The cause of the bridge's collapse, and whether the bridge's new construction techniques played a role, remained unknown on Friday as authorities launched investigations into the tragedy.

"Right now, we just want to find out what occurred, what caused this collapse to occur," said Miami-Dade Police Department director Juan Perez. "We're gonna have to start from the beginning, from contract, all the way to the end, when the incident happened."

Miami-Dade County Deputy Mayor Maurice Kemp added that no survivors were thought to remain in the rubble, which crushed at least eight vehicles like soft-drink cans, and that emergency efforts had switched to recovering the remaining victims' bodies.

"We want to ensure that this type of accident doesn't happen again locally or anywhere in this country," Kemp said.

More than 100 bridges around the US have been installed using the new construction technique, according to FIU's Accelerated Bridge Construction University Transportation Centre, which the engineers founded in 2010.

One of the most recent projects to use Accelerated Bridge Construction was the new Sacramento Wash Crossing on historic Route 66, along Arizona's western border, about 430 kilometres east of Los Angeles.

The road was closed for three and a half days to install the bridge last year. Officials estimated that traditional techniques would have required three months.

Saiid Saiidi, a professor of civil and environmental engineering at the University of Nevada, Reno, said that there was "nothing wrong inherently" with bridges built using Accelerated Bridge Construction techniques and that the chances of collapse are no greater compared with traditional methods.

"The internal connections and the way you put them together" are the same, Saiidi said, though he said the new methods can be more challenging. "You have to build them right, you have to design them right, so if anything, ABC bridges require a lot more care and the work is more carefully done."

The $US14.2 million ($18.4 million) bridge at FIU was designed by FIGG Bridge Engineers, and built by MCM Construction with help from funding from a US Department of Transportation TIGER grant, or Transportation Investment Generating Economic Recovery grant.

"This is an unprecedented event," FIGG said in a statement. "No other bridge designed by FIGG Bridge Engineers has ever experienced such a collapse."

FIGG was chosen to design the new Interstate 35W bridge in Minneapolis after the previous bridge, which had been built in the 1970s, collapsed in 2007, killing 13 people.

In a statement, MCM, the contractor, said it was "fully cooperating" with the NTSB's investigation and that it was "just heartbroken" for the victims. "We have been in business for more than 35 years, and safety has always been our number one priority," the company said.

The FIU-Sweetwater UniversityCity Bridge was initially built alongside Southwest 8th Street, an eight-lane thoroughfare, starting in spring 2017. Then, on March 10 - a Saturday - the bridge was lifted from its supports, rotated 90 degrees, and placed over 8th Street.

University officials initially hailed the process in a statement, saying the "method of construction reduces potential risks to workers, commuters and pedestrians and minimises traffic interruptions."

"FIU is about building bridges and student safety. This project accomplishes our mission beautifully," FIU President Mark Rosenberg said in a statement. "We are filled with pride and satisfaction at seeing this engineering feat come to life and connect our campus to the surrounding community where thousands of our students live."

The bridge was expected to open in early 2019 after further modifications.

After the collapse, Rosenberg filmed a video response promising a "thorough investigation" into the "tragic accident," saying, "It's exactly the opposite of what we had intended, and we want to express our deepest condolences to the family and loved ones of those who have been affected."

Florida Republican Senaor Marco Rubio tweeted that "the cables that suspend the #Miami bridge had loosened and the engineering firm ordered that they be tightened. They were being tightened when it collapsed today." Rubio did not offer further details, and neither FIGG nor MCM companies immediately responded to queries about the senator's claim.

The director of FIU's Accelerated Bridge Construction University Transportation Centre, Atorod Azizinamini, who was quoted in the university's news release initially praising the bridge, did not response to a request for an interview.

An FIU spokeswoman instead responded and directed the Los Angeles Times to an outside expert, declining to comment further.

Los Angeles Times