WASHINGTON, March 15 (Xinhua) -- At least four people have been killed by a pedestrian footbridge that collapsed near Florida International University (FIU) Thursday afternoon, local authorities said.

Miami-Dade County Fire Rescue Department Chief Dave Downey told a press briefing Thursday evening that nine victims were transported to hospitals "early on."

Miami-Dade County police said previously at least eight cars had been crushed under the walkway.

The incident, which took place early afternoon, involved a footbridge linking an FIU campus with the city of Sweetwater, where thousands of FIU students live.

Downey said over 100 rescuers have been dispatched to the scene, and will continue in "search and rescue mode" around the clock until it is determined that no more victims are trapped.

"The biggest obstacle for search and rescue is the stability of the structure," Downey said. First responders were cautious to lift bridge structures that had crashed on to passing cars, saying secondary damage may be caused if handled improperly.

Multiple sources confirmed that prior to the collapse, the bridge was being tested for strength, but it is unclear how the infrastructure was tested and what the result was.

According to witnesses, two workers were on the bridge when it collapsed. FIU President Mark Rosenberg told the press Thursday evening that constructors were conducting a stress-test on the bridge earlier Thursday, but the test result did not trigger any alarm.

"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," Florida Senator Marco Rubio said on social media.

Munilla Construction, one of the contractors for the project, issued a statement saying it is devastated by the "catastrophic collapse" and is "doing everything we can to assist."

The 950-ton cable-supported bridge spanned 174 feet (53 meters) over a six-lane expressway. The main span of the bridge was put in place on Saturday morning in less than six hours by adopting a rapid installation method. It is scheduled to open in early 2019.

The project cost 14.2 million U.S. dollars and was funded by the U.S. Department of Transportation.

The crashed bridge, which just days ago was hailed as a technological advance and a step toward meeting students' demand for easier access to campus, sent shockwaves through the FIU community.

Administrators have called for a swift investigation and Munilla Construction came under scrutiny as past allegations against the company's safety reputation surfaced.

In a statement, Florida Governor Rick Scott vowed to conduct a full investigation into the incident.

"If anybody has done anything wrong, we'll hold them accountable," he said.

The National Transportation Safety Board said that it was sending a team of 15 experts to examine the incident.

Munilla Construction pledged that it "will conduct a full investigation to determine exactly what went wrong and cooperate with investigators."

Facing questions on FIU's oversight, Rosenberg said all contractors involved in the project were "certified by the state" and that "this is one of the most intensely managed projects around" because federal funds had been invested in the project.

According to local media, Munilla Construction, a family business owned by six brothers that beat out three competitors to win the bridge contract, is an active construction business in Florida that has completed many successful projects, but has also been involved in lawsuits regarding the safety standard of its past work.

In October 2016, a member of the security personnel working at Fort Lauderdale-Hollywood Airport fell through a makeshift bridge that Munilla had built as it was renovating the airport.

"They built this makeshift bridge in the area where all the employees work, and it was poorly done," said Tesha Allison, a lawyer representing the victim.

The rapid construction method was also called into question. Louisiana Senator Bill Cassidy said the unconventional building method clearly does not work.

The FIU said Saturday the innovative installation method significantly reduced risks to workers, pedestrians and motorists and minimized traffic disruptions.