At least four people are dead and 10 others are injured after a bridge collapsed near Florida International University in Miami, officials say.
Rescue teams were still searching the rubble on Thursday night for victims trapped beneath the buckled footbridge.
The structure toppled over an eight-lane motorway on Thursday afternoon, crushing at least eight vehicles, according to police.
It is still unclear how many people were under the bridge at the time.
Miami-Dade Fire Chief Dave Downey said on Thursday night he could confirm at least four people were killed.
"This is going to be a long incident. We are going to have rescuers here through the night, if not longer," Chief Downey said.
At least 10 people were treated for injuries at the Kendall Regional Medical Center, including two who were in "extremely critical condition", according to Dr Mark McKenney, director of general surgery.
The 862-tonne, 53 metre bridge was erected on Saturday in just six hours over the highway, according to a story posted on the college website.
Witnesses told local media vehicles were stopped at a traffic light when the structure collapsed at around 1330 local time.
"We're working our way into the pile trying to create holes that we can actually physically see," Miami-Dade Fire Department Division Chief Paul Estopian told reporters.
A witness told ABC News that the screams coming from the cars were "terrifying".
"As soon as I looked outside, I saw dust flying everywhere," Tiona Page said. "I knew the bridge had collapsed."
Another witness, Damany Reed, told CBS News: "I heard a big kaboom... it sounded continuous. We look outside... we thought something had fallen but it was the bridge that collapsed. It was just surreal at that moment and pretty scary."
First-of-its-kind pedestrian bridge “swings” into place. “FIU is about building bridges and student safety. This project accomplishes our mission beautifully,” -President Mark B. Rosenberg. https://t.co/x8gPM9A4DG #worldsahead pic.twitter.com/mPEMeh2zmw— FIU (@FIU) March 10, 2018
"Just last week we were celebrating the expanse being completed and now we are here dealing with a tragedy," Sweetwater Mayor Orlando Lopez said.
Florida Governor Rick Scott and Senator Marco Rubio were at the scene on Thursday night, along with a team of specialists from the National Transportation Safety Board.
President Donald Trump tweeted on Thursday night he was monitoring "the heartbreaking bridge collapse".
Continuing to monitor the heartbreaking bridge collapse at FIU - so tragic. Many brave First Responders rushed in to save lives. Thank you for your courage. Praying this evening for all who are affected.— Donald J. Trump (@realDonaldTrump) March 15, 2018
The span, which connected the college to a student housing area in the city of Sweetwater, had long been requested by students and staff at the university so they could avoid the traffic below, according to the Miami Herald.
In August 2017, a student was hit and killed by cars while crossing the busy road.
Munilla Construction company (MCM), a family-owned contractor that helped build the bridge, tweeted "thoughts and prayers" for those affected by the tragedy.
FIGG Engineering said it also was behind the bridge project. Both firms said they would co-operate with investigators.
The university had touted the new swinging span on Twitter just days ago.
The 174ft cable-supported footbridge was to be publicly opened in 2019, local media say.
Costing $US14.2m, it was funded by the US Department of Transportation.
NBA basketball team the Miami Heat tweeted their support to the FIU community.
To all our friends in the @FIU community, we are thinking of you with heavy hearts.— Miami HEAT (@MiamiHEAT) March 15, 2018