(CNN) - Isabella Carrasco had just driven beneath a recently installed pedestrian bridge when she heard a thunderous crash.
She turned around to see what looked like the scene from an earthquake. The bridge near Florida International University was in ruins, having collapsed into a pile of concrete and twisted metal.
"The cars were completely crushed under it," Carrasco said.
"Quite frankly, I'm lucky to be alive. That could have easily been me and my boyfriend under that bridge had we not turned right and decided to stay at the light and gone a different direction."
Carrasco was one of several drivers and pedestrians near an intersection close to the Miami-area bridge when it collapsed Thursday, killing at least six people. Some saw the bridge turn to rubble in front of their eyes.
It "sounded like a bomb, like multiple bombs in one," Giovanni Hernandez told CNN affiliate WSVN. "It sounded like the world was ending, and when you look back, all you see is the bridge on the floor. It was awful."
Lynell Collins helped pull people from crumpled cars.
"After the whole thing broke, I was freaking out," Collins told CNN. "I got out of my car, and me and a few other people were sprinting over there. We started helping people whose cars were at least half crushed and whoever was easily saved. But we couldn't really go under any of the rubble because, at the time, the bridge -- the other half of the bridge is still kind of leaning upward right now."
Collins said one of the victims said his family was beneath the rubble but no one could help get them out.
"I'm sorry. I'm still shaken right now," he said.
Some first responders closed off the street and others tended to the injured with the help of medical students. A police officer shook her head when asked if she'd heard any response from people inside one car, Carrasco said.
A nearby worker told CNN affiliate WFOR that police held her back as she tried to help a boy trapped in the back seat of a car.
He was "just knocking on the window, full of blood, asking me for help," the woman said.
"It's insane," she said, her voice breaking with emotion, "because you have this person asking you for help and the police officer starts telling you, 'You can't. You can't go under the bridge to help him.' "
She later learned the boy didn't survive, WFOR reported.
Jose Mejia told the TV station that his efforts to rescue a man also failed.
"Went towards him thinking I could help him," Mejia said. "But I couldn't see his whole body. It was stuck under the bridge."
Through tears, Mejia described the experience as "very, very traumatic."
"I think a lot of people aren't going to forget this," he said. "At least I won't."
Aura Martinez and her mother were getting food at a nearby restaurant when they turned around and realized the bridge had collapsed.
The Florida International student said she initially didn't know what to do, but then her instincts kicked in.
"We ran out there and we tried to help people," Martinez told WFOR.
The bridge was being built to connect the school's campus to an adjoining neighborhood in Sweetwater, where many of the students live.
The structure's 950-ton main span had just been installed Saturday using an accelerated construction process meant in part to reduce the time that street traffic was halted.
"I was shocked because this thing they built, it had so much potential and everything, but it just literally killed people," Martinez told the TV station.
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