A newly installed pedestrian bridge over a major road in Miami collapsed, killing four people and trapping multiple cars below.
The walkway, which connected Florida International University to a student housing area, went up less than a week ago but was not yet operational.
"We have located up to four victims. Four deceased," Miami-Dade Fire Chief Dave Downey told a news conference.
Crews worked into the night at the scene of the disaster. Maurice Kemp, the area's deputy mayor, said the search for survivors had not been abandoned.
"Miami-Dade county and our partner agencies... have been working feverishly in the search and rescue mode to ascertain how many victims there are and rescue as many as we can," Kemp told journalists.
Miami-Dade Fire Rescue Division Chief Paul Estopinan said at least eight cars were trapped when the 950-ton (tonne) concrete bridge suddenly gave way.
Police detective Juan Carlos Llera said, "it sounded like an explosion. A huge bang."
"It looks like a disaster area. It looks literally like a bomb went off," Llera told AFP.
The bridge was suspended from cables that were determined to have loosened. While they were being tightened, the span collapsed, Florida Senator Marco Rubio wrote on Twitter.
The bridge had only been installed on Saturday, ahead if its planned opening in 2019.
Isabella Carrasco, who said she arrived on the scene just after the collapse, told CNN that some cars were completely crushed, and there was "just a lot of debris everywhere."
She said she saw one woman get out of a car that was "just nicked" and rescue personnel performing life-saving CPR on another person in the street.
Another shaken driver, Lynnell Collins, told CNN he was about to make a right turn when "the whole thing really just came down."
"After the whole thing broke, I was freaking out. 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."
He said he saw two trucks that were "completely crushed."
"We are shocked and saddened about the tragic events unfolding at the FIU-Sweetwater pedestrian bridge," Florida International University said in a statement.
"At this time we are still involved in rescue efforts and gathering information."
FIU had only recently been celebrating the installation of the bridge, which crossed a dangerous, heavily traveled section of highway that students said had been the scene of accidents.
It had been erected using an accelerated modular building method that enabled it to go up in the space of a day.
"We are stunned by today's tragic collapse of a pedestrian bridge," said FIGG Engineering Group, one of the partners involved in the walkway's construction.
"We will fully cooperate with every appropriate authority in reviewing what happened and why," the firm said in a statement.
"In our 40-year history, nothing like this has ever happened before."
"Continuing to monitor the heartbreaking bridge collapse at FIU - so tragic," President Donald Trump wrote on Twitter.
"Many brave First Responders rushed in to save lives. Thank you for your courage. Praying this evening for all who are affected," he said.
Bridge collapses in the United States are infrequent despite rising risks associated with aging infrastructure.
The deadliest this century was the 2007 collapse of an eight-lane bridge in Minneapolis, Minnesota, which killed 13 people.
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