At least six people have died after a pedestrian bridge collapsed on to a busy road in Miami.
Authorities said vehicles were crushed beneath massive slabs of concrete and steel in the incident on Thursday in which at least 10 people taken to hospital.
Search and rescue crews worked through the night, using dogs, search cameras and sensitive listening devices in a frantic bid to find survivors among the debris.
“Everybody is working hard to make sure we rescue anyone who can be rescued,” said Florida governor Rick Scott.
But Miami-Dade police director Juan Perez said hopes were dwindling as the hours passed.
“We know that there’s going to be a negative outcome at the end of the day,” Mr Perez said.
The 14.2 million dollar bridge crossed over a busy seven-lane road that divided a Florida International University campus from the city of Sweetwater.
The 950-ton span was installed on Saturday to great fanfare. It had not been opened when it collapsed on to vehicles waiting in traffic below, but two workers were on it.
Is was put to a stress test before it collapsed, and state and federal investigators are working to determine if that was a factor.
Senator Marco Rubio tweeted that the cables that suspended the bridge had loosened and the engineering firm ordered that they be tightened. “They were being tightened when it collapsed,” he said.
The main part of the 174ft span was assembled by the side of the road and had to be moved into place.
The “accelerated bridge construction” method was supposed to reduce risks to workers and pedestrians and minimise traffic disruption, the university said.
Miami-Dade County fire chief Dave Downey said: “We have to remove some of this piece by piece. It’s very unstable.”
Mr Scott added that an investigation will get to the bottom of “why this happened and what happened”. He said that if anyone did anything wrong “we will hold them accountable”.
National Transportation Safety Board chairman Robert Sumwalt III said a team of specialists was heading to Miami to begin an investigation.
Mr Rubio noted the pedestrian bridge was intended to be an innovative and “one-of-a-kind engineering design”.
The project was a collaboration between MCM Construction, a Miami-based contractor, and Figg Bridge Design, based in Tallahassee. Figg is responsible for the famous Sunshine Skyway Bridge across Tampa Bay.
Figg issued a statement on Thursday saying the company was “stunned” by the collapse and promising to co-operate with investigations.
“In our 40-year history, nothing like this has ever happened before,” the company’s statement said. “Our entire team mourns the loss of life and injuries associated with this devastating tragedy, and our prayers go out to all involved.”
MCM Construction Management, which was building the bridge, posted a message to the company’s Facebook page promising “a full investigation to determine exactly what went wrong”.
Robert Bea, a professor of engineering and construction management at the University of California, Berkeley, said it was too early to know exactly what happened, but the decision to use what the bridge builders called an “innovative installation” was risky, especially because it spanned a heavily travelled thoroughfare.
“Innovations take a design firm into an area where they don’t have applicable experience, and then we have another unexpected failure on our hands,” Mr Bea said after reviewing the bridge’s design and photos of the collapse.