"Several people were killed. I just don't know how many," Alejandro Camacho, a spokesman for the Florida Highway Patrol said in a telephone interview.
There were at least five to six vehicles crushed underneath the pedestrian crosswalk, he added.
Munilla Construction Management, which installed the bridge, in a statement on Twitter, said the bridge suffered a "catastrophic collapse causing injuries and loss of life."
God bless everyone, just watched this bridge fall from FIU and 109... just put up a few days ago. Hug your loved ones I’m shaking pic.twitter.com/wyNJ8LkwI9
— instabili-T (@TaionaBlunt) March 15, 2018
The bridge connects the university with the city of Sweetwater and was installed on Saturday in six hours over the eight-lane highway, according to a story that was posted on the university's website. It was 174 feet (53 m) long and weighed 950 tons.
The bridge was intended to provide a walkway over southwest Eighth Street, one of the busiest roads in South Florida. An 18-year-old female FIU student from San Diego was killed while trying to cross the street near FIU last August, according to local media reports.
Students at FIU are currently on their spring break vacation, which runs from March 12 to March 17.

The National Transportation Safety Board said that it was sending a team to investigate the bridge
White House spokeswoman Sarah Sanders said President Donald Trump is aware of the collapse.
"Our brave first responders are working feverishly to save lives," she said at a news briefing.
Florida Governor Rick Scott will head to the scene later in the day, his office said.
The National Transportation Safety Board said on Twitter that it was sending a team to investigate the bridge collapse.
Munilla Construction Management was founded in 1983 and owned by five brothers, according to its website. In addition to its Florida operations, the company also has divisions in Texas and Panama and employs 500 people.