A gunman killed 50 people and wounded 53 at a crowded gay nightclub in Orlando early Sunday in the deadliest mass shooting in American history. Here is what authorities know about how the attack developed so far:Before 2 a.m.: Omar Mateen, a resident of Fort Pierce, Florida, a city about 120 miles from Orlando, parked his van outside Pulse, a gay nightclub. He called 911 shortly before the attack and swore allegiance to the Islamic State.2:02 a.m.: Mateen entered the club armed with an AR-15-type assault weapon, a handgun and many rounds of ammunition, and opened fire, said John Mina, the Orlando police chief. The gunman was outside the club at some point after the initial shots were fired, the chief said, then returned inside, and "this did turn into a hostage situation."Police then marshaled forces, bringing in a SWAT team and an armored vehicle. At some point, Mina said, there was contact with the gunman inside. "There was some communication, but we are not going to release that right now," the chief said.It is unknown, Mina said, whether the gunman was killing others inside the club as the authorities prepared cautiously for a confrontation. "Any time we have a hostage situation, we're definitely going to use extreme measures to make sure we have enough personnel on the scene," Mina said.3 a.m.: A post on the club's Facebook page read, "Everyone get out of pulse and keep running." At some point, police heard from inside the building that at least 15 people were hiding in a restroom.5 a.m.: Police began an attempt to rescue the hostages. They detonated two explosives to distract the gunman. Eleven officers entered the club, and shots were again exchanged. Mateen was killed. One officer was shot in the head, but his Kevlar helmet saved him from serious injury, Mina said. At least 30 hostages were found alive.The gunman was armed with an AR-15-type semi-automatic weapon and a handgun, Mina said.The AR-15-type semi-automatic weapon is the same type of gun used in the mass shootings in Aurora, Colo.; Newtown, Conn.; and San Bernardino, Calif. Gun control advocates contend that semi-automatic weapons like the AR-15 are a logical choice for mass shootings because of their ability to rapidly fire multiple high-velocity rounds. Defenders of the firearm say it is misguided to blame a gun that is used by millions of owners across the country in a responsible manner. The weapon is legal to buy in most states, including Florida. In 1994, Congress passed an assault weapons ban that prohibited manufacturing AR-15 for civilian sale with large-capacity magazines, bayonets or pistol grips. The ban limited, but did not end, sales of AR-15s. The weapons ban expired in 2004.