Witnesses of the bridge collapse at Florida International University in Miami have described a cloud of dust being sent into the air alongside a thunderous boom as the newly built, 950-tonne pedestrian structure smashed into the street below, trapping several vehicles.

The resulting scene, according to those near the site, was chaotic. People could be heard screaming from the trapped cars underneath the rubble of the bridge as emergency responders arrived and began to search for survivors.

“The sound was thundering,” Piqué Philips, who was driving on the fourth floor of a parking garage adjacent to the bridge, told The Independent. “Like an extremely loud boom.”

Police on the scene told the Miami Herald that as many as six people were killed in the collapse, and Miami-Dade Fire Rescue says that at least eight others were taken to the hospital on trauma alert. As many as six cars were caught beneath the collapsed structure.

Steven Ippolito Jr, a meteorologist with the National Weather service in Miami, says that he was a couple blocks down the street in traffic, waiting to turn right, when the bridge went down, seemingly without notice.

“All of a sudden, the bridge just fell,” Mr Ippolito told The Independent. “There was law enforcement [police officers] right next to me, and I don’t even think he knew how to react as his lights didn’t go on, nor did he rush to see the scene immediately after.”

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Tonia Page, a witness who spoke with ABC News, said that the screams were “terrifying”.

“As soon as I looked outside, I saw dust flying everywhere,” Ms Page said. “I knew the bridge had collapsed.”

While it was not immediately clear what caused the bridge collapsed, nearby construction workers speculated that it was an “engineering failure,” Monique O Madan, a reporter with the Miami Herald, tweeted, alongside a picture of men in orange vests and hard hats looking on at the toppled bridge. Ms Madan later tweeted that she had spoken to a woman whose car had been smashed by the bridge.

"Katrina Collazo was at a red light when she said she felt something like small rocks fall on her car," the tweet reads. "As she turned around, the whole back seat was smashed and the car next to her, buried."

Florida International University released a statement saying that they are “shocked and saddened”, and that they plan on continuing the efforts to determine what went wrong.

“We are shocked and saddened about the tragic events unfolding at the FIU-Sweetwater pedestrian bridge,” the statement read. “At this time we are still involved in the rescue efforts and gathering information. We are working closely with authorities and first responders on the scene. We will share updates as we have them.”

The bridge was described as an “instant bridge” before its collapse, because it was installed in a single morning on Saturday. The bridge, which had not yet opened, cost $9 million and was funded through a 2013 US Department of Transportation grant.