The incident happened on 64th Street between Madison and Park avenues just before 5 p.m.
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Seconds after the giant SUV came to a stop onlookers ran to help.
A driver and passenger were inside the SUV, and a 41-year-old woman was gravely hurt on the sidewalk.
"It's pretty bad, pretty bad. She was on the floor laying down, she wasn't moving, she was bleeding, gushing blood," said Carlos Medina, a witness.
Bike messenger Medina had been riding in the lane next to the Chevy Suburban on Madison.
"I was like holy moly," said Mike Briggs, a tourist and witness.
Briggs was just doing some shopping when he saw the crash happen.
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"I saw him run this light, he was doing 50-60 miles per hour, and then I saw him swerve and go on the sidewalk," Briggs said.
"And that's when he kept taking up the sidewalk and made his way up to here," Medina said.
"He hit so many things going through there and it was just boom, boom, boom, boom, boom," Briggs said.
As cops tried to get to the driver, they found him slumped over the steering wheel.
"He was having an attack, he was having a seizure attack," Medina said.
Authorities are trying to figure out if he had some kind of medical episode that caused all this chaos.
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Investigators spent hours talking to eyewitnesses, including one man driving his own Suburban at the time of the crash, it was damaged by a flying street sign.
And late Thursday night, as a flatbed picked up the wrecked truck, onlookers were marveling that the carnage wasn't even worse.
"Everybody was just looking staring, it was really shocking," said Olga Rosenbaum, a witness. "I could have been walking there at same time, I walk there all the time back and forth."
The 41-year-old woman was rushed to Cornell Medical Center in critical condition.
The driver of the SUV was extricated from the vehicle and taken to an area hospital in critical condition.
One other passenger suffered minor injuries.