It was Valentine's Day across America.
At an all-American high school in a green and placid Broward County suburb strung with waterways and golf-course communities, that innocent fact would take on dreadful significance well before the day was out.
Between classes at Marjory Stoneman Douglas High, near the sharp, straight line where suburban Parkland meets the Everglades, students exchanged chocolates, flowers and cards.
It was another full, demanding day at the A-plus public school, which at 3,200 students is larger than many colleges. One history class would have a lesson on the Holocaust. There would be drama class, study hall, group projects to work on. The Junior ROTC would do parade drills. The school's award-winning color guard would work on its routines.
At the gym complex, head football coach Willis May received recruiters from a small Massachusetts college in his office.
Early that morning, not far from the school's expansive campus, a troubled young man named Nikolas Cruz was awakened for school by his host, James Snead. Snead and his wife, whose son was one of Cruz's few friends, took in the young man after he lost his mother — his father had died previously — to a fatal illness in November. Unlike the Sneads' son, though, Cruz wasn't attending Stoneman Douglas any longer. He had been expelled for bad behavior and was enrolled in an adult education school to get his GED.
But Cruz, who is 19, told Snead he wouldn't be going to school that day at all.
"He said something to the effect of, 'It's Valentine's Day. I don't go to school on Valentine's Day,' " family attorney Jim Lewis told CBS News. "They didn't think anything of it."
Snead went off to work and his son to Stoneman Douglas. And then Cruz apparently put into motion a plan he is believed to have been hatching for some time. He got into a locker where the Sneads had him secure his semiautomatic AR-15 rifle — among the possessions Cruz brought to their house after his mother died.
Cruz packed the AR-15 into a soft-sided rifle case, Broward Sheriff's officers say. He put extra ammunition in his backpack. Then, in a coldly canny move, he donned a maroon Stoneman Douglas Junior ROTC polo shirt.
Cruz used to be a member, and he knew that on Wednesdays the ROTC kids wore their polos to school, said Zackary Walls, a senior at Douglas High and JROTC member. The polo would allow Cruz to blend in when he went back to the school.
This account of what happened during one of the nation's worst school gun massacres is pieced together from a police timeline that is based in part on security camera footage from inside and outside the school, and the descriptions and recollections of those who lived through that terrible afternoon.
At 2:06 p.m., Cruz got into an Uber car. He was dropped off at Stoneman Douglas 13 minutes later and strolled onto the sprawling campus, apparently unchallenged. Cruz may have picked the time, just before the dismissal bell, because school security would be unlocking gates around the campus. One student said Cruz might have entered through a "senior gate" that leads to a large parking lot on the north side of the school.
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Facing the lot is the three-story Freshman Building, so called because it once housed mostly ninth-graders, but officially designated as Building 12. It holds 33 classrooms.
Cruz went in through the east stairwell at the ground floor of the building, the only one at the school with enclosed hallways. When he came out, two minutes after arriving at the school, the rifle was out of the bag. Cruz began firing.
First he shot into one classroom, then the one across the hall and then the next one after that. He then doubled back to the first two rooms before shooting up a fourth.
It was total terror and confusion in Building 12.
Survivors' accounts suggest that Cruz shot into the classrooms from the hallway, firing through doors and windows, rather than entering the rooms.
Rebecca Bogart, 17, was listening to the lecture about the Holocaust in a classroom on the first floor of Building 12 when she heard gunshots.
"We all took cover and hid," said Bogart, who said she squeezed under her teacher's desk. "We were just trying to stay calm. Everyone tried to stay quiet."
Moments later, gunshots hit and shattered a window separating the classroom from the hallway.
"So I knew it was real. This was happening," she said. "This is something you see on the news and never expect to happen."
Four kids in the class were injured, Bogart said.
Samantha Mahecha, 14, was in study hall in a classroom next to the entrance of the building when she, too, heard gunshots. Her classmates ran to one corner of the room to hide behind the teacher's desk.
The teacher was worried about a group of students she had allowed to leave the classroom before the shooting started, Mahecha said. Students were allowed to come and go during study hall to work on group projects. One student had been in the hallway working on a project when the shooting started. Two other boys had been on their way back to the classroom.
Mahecha heard glass shatter as a bullet broke an interior window nearby. Then she heard screaming in the hallway. The two boys tried to get inside the classroom, but they didn't make it in time. Later she would learn that three of the students who had left the class to work on projects were among the dead.
Cruz took the western stairwell up to the second floor and shot into a single classroom there, before climbing to the third floor, where witnesses say he continued his deadly spree.
The deadliness of Cruz's rampage may have been exacerbated by something that happened early on: Shortly after the shooting commenced, a fire alarm went off. It's unclear whether Cruz deliberately set off the alarm. Broward Schools Superintendent Robert Runcie said later it was likely tripped by smoke from the firing of the assault rifle in the closed hallway.
The fire alarm put students and teachers directly in Cruz's path before it dawned on anyone what was really going on. That was followed shortly afterwards by a lockdown order issued once administrators realized they were dealing with every school staffer's worst nightmare: a live shooter on campus.
One staff member spied Cruz walking in, recognized him as a known troublemaker who shouldn't be on campus, and contacted the office with a warning.
In a sad new commonplace in American life, schools now routinely drill students and staff on how to respond in case of an active shooter. The procedure, dubbed Code Red, calls for shutting and locking classroom doors and not letting anyone in. Students and staff at Stoneman Douglas were well drilled in what to do in a Code Red.
Instructions on what to do in case of a fire alarm, however, call for the opposite. Fresh off a recent fire drill — there had been one earlier that same day — students and faculty did what they were required to do when they heard the fire alarm go off again: They began leaving their classrooms, some into the line of fire.
Shortly after the fire alarm rang, football coach May said, assistant principal Winfred Porter broadcast a Code Red over the school public-address system.
The prompt Code Red and alert reactions by students and staff likely saved many lives. But the contradictory instructions also inevitably sowed confusion and left some students and teachers stranded in hallways, unable to get back into locked classrooms. Many made the choice to flee for their lives instead.
Some teachers then began ordering students back into classrooms and locking them down. May said he locked down the players and visiting coaches in his office, then locked down the whole P.E. area, which sits catty-corner to Building 12.
Panic-stricken and unsure what to do, students began running in all directions, not just in Building 12 but also in nearby classrooms where the bangs from Cruz's shots reverberated sharply. Some ran back into classrooms or hid in bathrooms or closets. Others ran outside. Faculty and staff herded some students into the auditorium, which flanks the main school entrance on the east side of campus.
Junior ROTC captains ushered 60 cadets and other students back inside a classroom when they recognized gunfire, and deployed large Kevlar panels in front of them for protection after barricading the door with a table, said Colton Haab, one of the officers in the program. The projectile-resistant Kevlar, the same material found in bulletproof vests, is used in the ROTC air-rifle marksmanship program — where Cruz, a former member of the program, apparently sharpened his shooting skills.
"Once we realized it wasn't a drill, everyone hid behind the bulletproof barrier," said JROTC member Gatlin Alvord, 14.
Nicholas Coke was sitting in his last class of the day, English, when the fire alarm went off for the second time Wednesday. He left his bag and walked out of the classroom. Then, he said, he heard several pops.
"I wasn't going to stick around and find out what was going on," he said.
He ran, as did his fellow students. Coke said they had spent a good part of the year preparing for this kind of situation, and at first he thought it was a simulation. But putting plans into action turned into chaos, he said. He described people dashing outside and jumping fences to the adjacent middle school, just to the west of the high school, and then continuing to run.
"We were told it was a fire drill, but as we were coming down the stairs, an administrator started yelling that it was a Code Red," said sophomore Jake Malka, 15. "So everyone started running back upstairs. And kids were getting, like, trampled."
Their teacher was supposed to leave the door to the classroom open during the fire drill "in case something like this happened," Malka said. But she had left and locked the door, leaving the students marooned in the hall, he said.
"We all had to run into a different teacher's classroom. There wasn't enough room, so me and a few friends had to get into a closet. And all we could hear was the radio. Through the intercom they were telling us what happened."
Gabriella Figueroa, 16, was in geometry class in the 700 building, which is next to Building 12, when the fire alarm went off. She and some others went out in the hallway and she heard a popping sound. She tried getting back into her class, but the teacher shut the door and locked it, she said.
So she banged on the classroom next door, and the teacher let her and her friends in. "We were so thankful," she said.
The students in Scott Beigel's geography class filed out of the third-floor classroom in Building 12 and headed down the stairs at the sound of the fire alarm. When they heard shots, they ran back upstairs. Beigel ushered them inside the classroom and the students hid under their desks, some said. Some accounts have Beigel shutting the door, then reopening it to admit students who had been left outside.
Mia Sanchez, 16, said Beigel was at the doorway, ushering in other students, when he suddenly yelled in the direction of the approaching shooter: "There is nobody in the classroom!" Then Sanchez saw her teacher get shot. Beigel fell in the doorway, and the students were unable to close the door.
They cowered under their desks for what felt like an hour, Sanchez said, staying silent because the door remained ajar. At one point smoke filled the room, she said. Sanchez isn't sure if the shooter tried to enter the room.
There were other stories of heroic action by staff and students. An assistant football coach and security guard, Aaron Feis, was killed while trying to shield students, with some reports saying he stepped in front of Cruz's bullets. Freshman Peter Wang was seen holding a door open for other kids to exit through when he, too, took a fatal bullet.
Read More: The victims of the Parkland school shooting
Junior Shay Makinde was lauded by his peers for pushing two classmates into a room as the gunman approached. Makinde, who survived unscathed, was unable to save a third, his best friend, Joaquin "Guac" Oliver, who was shot in the head.
Makinde, 16, said he was in the second classroom Cruz shot up. After the fire alarm, he said, half of his class had run away while the others walked, unaware of what was going on.
"As soon as we heard gunshots, we all tried to help each other," Makinde recalled, fighting back tears at a vigil the day after the shooting.
Earlier that Valentine's Day, teacher Ernest Rospierski said faculty members were told during a staff meeting that there would soon be a Code Red lockdown drill, but they hadn't been told the date.
When the fire alarm rang, he and his students filed out of their third-floor classroom and made their way toward the stairs. Then Rospierski heard the sound of gunfire. He realized this was no drill.
"Turn around, go to your classrooms!" he yelled.
Students started running back toward him and he tried to steer them into open classrooms, Rospierski said. By the time he reached his classroom door, the shooter was at the end of the hall.
Rospierski was locked out of his classroom, so he pushed eight kids into a shallow alcove by the classroom door and desperately tried to keep them out of the line of fire. He didn't know it at the time, but the gunman was a former student who had taken his freshman World Geography class five years before.
Rospierski felt a bullet graze his cheek. The shooting then stopped as the gunman reloaded. Rospierski said he took advantage of the pause to push his students toward the stairway so they could escape. He followed closely behind. A girl was lying by the stairs. He checked for a pulse, but couldn't find one.
Then the shooting resumed. With his students on their way down the stairs, Rospierski ran into the nearby bathroom to hide.
Just as abruptly, Cruz stopped shooting again, this time for good. It had been a scant six minutes since he had first opened fire.
Police say Cruz dropped his weapon and backpack on the third floor of Building 12 and joined the stream of terrified students making their way downstairs. In the turmoil, he managed to slip away.
When he reached the ground floor, police say, Cruz ran west towards the school tennis courts, cut south and crossed an athletic field with other fleeing students, then turned back west. The zigzag path was probably necessary to get off the school campus, which is bordered on the south and west by canals, and reach a main road that would provide an escape route.
Twenty minutes later, Cruz had made his way into an adjacent Walmart, where he went in and bought a drink at the Subway counter. Ten minutes later, he walked into a nearby McDonald's, sat down briefly — police did not say whether he ordered anything — then walked back out.
Forty minutes later, at 3:41, Coconut Creek police officer Michael Leonard, cruising the area looking for the suspect, spotted a lone teenager walking near the entrance to the Pelican Pointe subdivision. The teen's clothing matched the description Leonard had heard on his police radio. The officer stopped his cruiser and detained the unarmed teen, who surrendered without resistance.
Back at Stoneman Douglas, long after the shots had ceased, fear and uncertainty still reigned. Students who had made it out had been shuttled to a nearby hotel to wait for their parents. The neighborhood around the school was a madhouse, with cars backed up in every direction and blocking every road for miles amid a massive law-enforcement presence.
On campus, students and teachers were stuck in closets and classrooms for one hour, two hours, fearful of stepping out, as police painstakingly searched and cleared school buildings.
Many followed the news on their cellphones or alerted parents and friends they were safe.
Figueroa, who had been locked out of her geometry class near Building 12, stayed inside a hot book closet with a group of other students for about two hours, until police let them out.
"I was panicking," she said. "I couldn't believe what was happening."
Sarah Crescitelli, a 15-year-old freshman, was locked inside the bathroom of her theater class with 40 other students for about two hours. Earlier, in the dark, as shots ran out and students around her wept, she had composed a text to her parents: "If I don't make it I love you and I appreciated everything you did for me."
She said afterwards: "People were trying to comfort each other, saying we are going to get out of here, that we were going to be all right. I'm shocked. I never thought that could happen."
Mahecha said her classroom was the first in the building to be evacuated when the police arrived.
As kids were escorted out of Building 12 by police, the evidence of Cruz’s cruel handiwork was everywhere. Floors were covered in blood, they said. The bodies of their teachers and fellow students still lay where his bullets had felled them.
"On the floor you see kids bleeding, just a lot of blood everywhere," said Bogart, who had been in the class on the Holocaust. "I am so grateful to be living right now."
By 6:30 p.m., police had tallied up the terrible toll. Broward Sheriff Scott Israel announced that Cruz, who confessed, had murdered 17 people in cold blood on Valentine's Day at Stoneman Douglas, and injured 15 others.
He said 12 people had died inside the school building, and two in the hospital. He also said two died outside and one in the street.
Two days after, that was still puzzling investigators. As far as they could determine, Cruz never fired outside Building 12. They have a couple of possible explanations: Those victims were struck by bullets that flew outside through windows as Cruz sprayed classrooms with the powerful rifle. Or some injured people made their way outside before collapsing.
The death toll far surpassed the number of dead in another Valentine's Day massacre.
In 1929, mobsters opened fire on rivals in a Chicago gangland war. The number of dead in one of the most notorious mass murders in U.S. history: Seven.