“This has turned from a rescue to a recovery operation,” said Det Alvaro Zabaleta of Miami-Dade police department on Friday.
Emergency services had been racing to find survivors in the rubble using high-tech listening devices, sniffer dogs and search cameras.
“We have to remove some of this piece by piece. It’s very unstable,” said Dave Downey, the fire chief of Miami-Dade county.
The $14.2m (£10.2m) bridge was supposed to open in 2019 as a safe way to cross a busy road between the campus of Florida International University (FIU) and the Sweetwater district, where many students live.
The governor of Florida, Rick Scott, and the junior senator for the state, Marco Rubio, joined other officials at the scene. Rubio said the public and the families of the dead and injured deserve to know “what went wrong”.
Scott said an investigation would get to the bottom of “why this happened and what happened,” and that if anyone did anything wrong, “we will hold them accountable”.
Robert Sumwalt, the chairman of the National Transportation Safety Board, said a team of specialists would begin its investigation Friday morning.
Rubio, who is an adjunct professor at FIU, said the bridge was intended to be an innovative and “one-of-a-kind engineering design”.
In models and drawings of the structure, the bridge has a tall, off-centre tower with supporting cables attached to the walkway. When the span collapsed, the main tower had not yet been installed, and it was unclear what builders were using as temporary supports.
An accelerated construction method was supposed to reduce risks to workers and pedestrians as well as minimise disruption to vehicles travelling below, FIU said. The university has long been interested in this kind of design; in 2010, it opened an Accelerated Bridge Construction Centre to help in the development of the method.
The pedestrian bridge was a collaboration between Miami-based MCM Construction and Figg Bridge Design, which is based in Tallahassee. Figg is responsible for the Sunshine Skyway Bridge across Tampa Bay.
In a statement on Thursday, Figg said the it was stunned by the collapse and that it would cooperate with investigations.
“In our 40-year history, nothing like this has ever happened before,” the company 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 said it would participate in “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 why the structure collapsed, but the decision to use what the bridge builders called an “innovative installation” over a busy road was risky.
“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,” he said after reviewing the bridge’s design and images of the collapsed structure.
FIU staff and students, along with Sweetwater and Miami-Dade officials, held a “bridge watch party” last Saturday, when the span was lifted from its temporary supports, rotated 90 degrees across the eight-lane road and lowered into its permanent position.
Mark Rosenberg, the president of the university, said tests were being done on Thursday. Authorities said two construction workers were on the bridge when it collapsed. It is unclear what the tests were or whether they contributed to the failure of the structure.
“This bridge was about goodness, not sadness,” Rosenberg said. “Now we’re feeling immense sadness, uncontrollable sadness. And our hearts go out to all those affected, their friends and their families. We’re committed to assist in all efforts necessary, and our hope is that this sadness can galvanise the entire community to stay the course, a course of goodness, of hope, of opportunity.”
