Here's everything we know about what happened.
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THE COLLAPSE
Authorities were called to the scene of the accident around 1:30 p.m. local time Thursday. Upon arrival, they found eight cars trapped beneath the collapsed span and immediately launched a search-and-rescue operation that included more than 100 first responders from multiple agencies. Police dogs and heavy machinery are aiding the investigation.
The victims have been identified as FIU student Alexa Duran, 57-year-old Oswald Gonzalez, 53-year-old Alberto Arias and 60-year-old Rolando Fraga Hernandez, 60. Police also released the identity of victim Navarro Brown, who died at a hospital shortly after the accident.
In addition to Brown, nearly a dozen other people were taken to the hospital during the course of the rescue operation, according to Miami-Dade Fire Rescue. Those victims' conditions are not immediately known.
Multiple bodies have been removed from the scene of the accident, and authorities said Friday that they expect to find more as they continue their investigation.
In video obtained by ABC News, eyewitnesses in the upper floor of a building across the street recall seeing people beneath the bridge at the time of the collapse. One witness said she saw a worker atop the bridge at the time of the collapse, but it's unclear if that construction worker was among the injured or dead.
Others said the collapse was sudden and produced an audible boom as the bridge hit the ground below, with one person likening the moment of impact to an earthquake.
"God bless everyone involved. I hope everyone comes out as safe as possible," one frightened witness said.
THE BRIDGE
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Collapsed Florida International University bridge was just installed days ago
The span of the FIU-Sweetwater UniversityCity Bridge had been assembled just days before the collapse.
The 950-ton, 174-foot portion that later failed was assembled on the side of the road before being swung into place and connected with support towers on the side of the road on Saturday. It was hailed by Jorge Munilla, president of engineering firm MCM, as the largest bridge in the United States to be constructed that way.
That accelerated method was supposed to reduce risks to workers and pedestrians and minimize traffic disruption, Florida International University said before the collapse.
"The design is incredible. It's going to be one of those game changers," Miami-Dade County Commissioner Jose Diaz said of the engineering behind the structure.
The bridge spanned 8th Street, a major seven-lane thoroughfare that runs from downtown Miami to the Everglades, connecting Florida International University to the nearby Sweetwater neighborhood. In August 2017, a university student was killed crossing the road that the bridge was supposed to span.
The $14.2 million bridge was expected to open to foot traffic in early 2019.
THE INVESTIGATION
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It's not immediately clear what caused the bridge to collapse. In addition to local agencies, the National Transportation Safety Board has sent investigators to the scene. Florida Gov. Rick Scott and Sen. Marco Rubio also visited the scene.
According to NTSB investigators, workers were trying to strengthen a diagonal member on the bridge when it collapsed. Robert Accetta, the investigator-in-charge for the NTSB, said crews were applying post-tensioning force, but investigators aren't sure if that's what caused the bridge to fall.
Engineer warned DOT about crack days before bridge collapse
An engineer left a voicemail two days before the collapse to say some cracking had been found at one end of the concrete span, but the voicemail wasn't picked up until after the collapse, Florida Department of Transportation officials said Friday. It's not yet clear if that cracking directly contributed to the bridge's failure.
Local authorities have asked the public to avoid the area while the investigation continues.
MCM, the Miami-based construction management firm who won the bridge contract, said in a statement that it was "devastated and doing everything we can to assist."
Pedestrian bridge collapses at Florida International University
"We will conduct a full investigation to determine exactly what went wrong and will cooperate with investigators on scene in every way," the company wrote on its Facebook page.
FIGG, the engineering firm that worked with MCM, said in a statement it was "stunned by today's tragic collapse" and also pledged to cooperate with authorities.
"In our 40-year history, nothing like this has ever happened before. Our entire team mourns the loss of life and injuries associated with this devastating tragedy, and our prayers go out to all involved."
FIGG was fined in 2012 after a 90-ton section of a bridge it was building in Virginia crashed onto railroad tracks below, causing several minor injuries to workers. The citation, from the Virginia Department of Labor and Industry, said FIGG did not do the proper inspections of the girder that failed and had not obtained written consent from its manufacturer before modifying it, according to a story in The Virginian-Pilot.
The Associated Press and ABC News contributed to this report.