At least six people are dead after a new pedestrian bridge collapsed onto a highway at a university in Miami, crushing at least eight vehicles under massive slabs.
Cars were trapped underneath the structure, while several people were loaded into ambulances as authorities launched search and rescue missions.
Emergency personnel with sniffer dogs searched for signs of life amid the wreckage of concrete and twisted metal on one of the busiest roads in South Florida.
Firefighters have pulled out at least four deceased people from the rubble, according to Miami-Dade fire chief Dave Downey said at a Thursday evening press conference.
At one point, police requested television helicopters to leave the area so rescuers could listen for any sounds of people crying for help from beneath the collapsed structure.
The bridge crosses over a busy seven-lane road highway that divides Florida International University campus from the city of Sweetwater.
At least eight people were taken to the trauma centre at Kendall Regional Medical Center, a hospital source said. The condition of the patients is unknown.
Witnesses have described the collapse as feeling like an "earthquake".
"It felt as if there was an earthquake so me and my roommate jumped up and looked outside, and as soon as I looked outside I saw dust flying everywhere and I knew the bridge had collapsed," nearby resident Tiona Page told US broadcaster ABC.
Another witness said: "It was like an explosion, like a bomb."
In one video from the scene shared on Twitter, a witness can be heard saying a worker was standing on top of the bridge as it collapsed.
Miami police warned people to avoid the area.
The Miami Herald reported the bridge was installed on Saturday morning. It was due for completion in 2019.
"We heard a loud bang behind us... and we looked back and the bridge had completely collapsed," University of Miami student Isabella Carrasco told CNN.
Ms Carrasco said she could see at least five cars which had been totally crushed by the bridge.
Florida International University issued a statement saying it was "shocked and saddened about the tragic events unfolding on campus."
"At this time we are still involved in rescue efforts and gathering information," the university said.
"We are working closely with authorities and first responders on the scene."
According to a press release from the university, the 950-ton bridge was installed "in a few hours" using "accelerated bridge construction" methods to minimise "potential risks to workers, commuters, and pedestrians and minimizes traffic interruptions."
Munilla Construction Management, one of the companies that constructed the bridge, said in a statement that it will conduct a "full investigation to determine exactly what went wrong" and that it will cooperate with investigators "in every way" possible.
"The new UniversityCity Bridge, which was under construction, experienced a catastrophic collapse causing injuries and loss of life," the company said.
"MCM is a family business and we are all devastated and doing everything we can to assist.
"We will conduct a full investigation to determine exactly what went wrong and will cooperate with investigators on scene in every way."