Gary Harrington saw the Nissan Altima stop abruptly in the intersection, turn and drive the wrong way on Gandy Boulevard.
Harrington shouted. He honked. He waved his arms.
"Nothing was going to stop her," said Harrington, 50. "She was oblivious to everything going on around her."
About a half mile down the road, the Nissan slammed into a sport utility vehicle, then a van and a sedan. The impact split apart the Nissan.
Its driver, Ngoc-Huong Tran, 50, of St. Petersburg, was pronounced dead at the scene.
Four other people, including a young child, were taken to Bayfront Medical Center after Tuesday's chain-reaction crash. Their injuries were not life-threatening, the Florida Highway Patrol said.
The 10:09 a.m. accident closed the westbound lanes of Gandy Boulevard for several hours.
"The cause is going the wrong way, but we don't know why," said FHP Sgt. Kristina Quenneville.
Some witnesses told FHP investigators that Tran had been traveling in the proper lanes as she drove east on Gandy Boulevard.
But for some reason, she got in a lane to make a left turn at the Grand Avenue intersection and drove east in a westbound lane of Gandy.
Harrington, a witness in a westbound lane, said he and his passenger watched the Nissan straddle the grass median.
"We never saw a brake light," Harrington said.
FHP investigators said the Nissan first struck a Hyundai sport utility carrying a child and two adults. The Hyundai flipped and landed in the median.
The Nissan then hit a Chevrolet van owned by the city of St. Petersburg. The Nissan spun and split in half, the engine landing 30 feet away.
The Nissan was then hit broadside by a Toyota Avalon from Massachusetts. The driver's side of the Nissan was crushed like an accordion.
_ Times researcher Caryn Baird contributed to this report.