MIAMI (CBSNewYork/AP) — A newly-installed pedestrian bridge collapsed Thursday afternoon near Florida International University in the Miami area.

Authorities said at least four people were killed when the structure fell around 1:30 p.m., crushing eight cars.

More: CBS News | CBS Miami

The collapse triggered a panic among witnesses. Some, like Isabella Carrasco, had just driven beneath the bridge.

“We had heard a loud bang. At first, we thought we had hit something, and when we looked around, we realized the bridge had collapsed on multiple cars. I didn’t see any pedestrians walking on the bridge,” she said. “When we looked around, we saw that it was much, much worse.”

More than 100 firefighters and K-9 units raced through rubble, searching for survivors. Authorities ordered helicopters and other emergency vehicles away from the scene in order to listen for possible cries from people trapped below.

“We’re going to continue to search this pile until we’re sure there are no other victims,” Dave Downey, of the Miami-Dade fire department, said.

The 950-ton span connecting Florida International University to the city of Sweetwater had just been installed Saturday in a huge celebration.

The university tweeted out a time-lapse of the installation.

“We are shocked and saddened about the tragic events unfolding at the FIU-Sweetwater pedestrian bridge. At this time we are still involved in rescue efforts and gathering information. We are working closely with authorities and first responders on the scene,” FIU said in a statement.

“What was soon to become an iconic staple part in the connectivity between the city and the university, has actually turned out to be a national tragedy,” said Sweetwater Mayor Orlando Lopez.

The $14 million project was being constructed using something called the “accelerated bridge construction method,” which is supposed to make it safer for workers, pedestrians and commuters. The bridge was designed to withstand a Category 5 hurricane.

A YouTube video depicts what the completed design would have looked like.

CBS News has learned workers may have been performing stress tests on the bridge at the time of the accident.

“It’s never wise to be working on a bridge that isn’t fully secured with traffic underneath it,” said Sam Schwartz, former chief engineer for the City of New York. “I’ve worked on prefabricated bridge structures. This is the first that I know that’s collapsed.”

Late Thursday night, Sen. Marco Rubio offered a possible explanation on Twitter.

“The cables that suspend the #Miami bridge had loosened & the engineering firm ordered that they be tightened. They were being tightened when it collapsed today,” he posted.

White House press secretary Sarah Huckabee Sanders said the president was monitoring the situation and would offer whatever support was needed.

The National Transportation Safety Board has launched an investigation. 

FIGG, the engineering company that designed the bridge, said it was “stunned by today’s tragic collapse.”

“Our deepest sympathies are with all those affected by this accident. We will fully cooperate with every appropriate authority in reviewing what happened and why,” the statement continued. “In our 40-year history, nothing like this has ever happened before. Our entire team mourns the loss of life and injuries associated with this devastating tragedy, and our prayers go out to all involved.”

Munilla Construction Management, the company that was constructing the bridge, called it a “terrible tragedy.”

“The new UniversityCity Bridge, which was under construction, experienced a catastrophic collapse causing injuries and loss of life. MCM is a family business and we are all devastated and doing everything we can to assist,” its statement read in part.

CBS News has also learned both companies have been accused of unsafe practices in the past, where there were injuries but no deaths.

“I know we’re going to all want to do our best to find out exactly what happened here, and we’ll do that in a transparent matter. If anybody’s done anything wrong, we’ll hold them accountable,” Florida Gov. Rick Scott said.

According to its website, FIGG engineering also designed a couple big projects in the Tri-State Area, including the AirTrain railway bridge at John F. Kennedy International Airport and the Victory Bridge in Sayreville, New Jersey.

(© Copyright 2018 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)