The walkway, which connected Florida International University to a student housing area, was built less than a week ago
Four killed as footbridge collapses in Miami
A new 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, was completed less than a week ago.
"We have located up to four victims. Four deceased," Miami-Dade fire chief Dave Downey said.
Maurice Kemp, the area's deputy mayor, said the search for survivors had not yet 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," Mr Kemp said.
Miami-Dade fire rescue division chief Paul Estopinan said at least eight cars were trapped when the 950-tonne concrete bridge suddenly gave way.
Police detective Juan Carlos Llera said, "it sounded like an explosion. A huge bang".
"It looks literally like a bomb went off," Mr Llera said.
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 resuscitating another person in the street.
Another driver, Lynnell Collins, said 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 lorries 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."
Donald Trump, the US president said he is monitoring the events.
Bridge collapses in the US 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.