Search and rescue missions were underway. Eight people were taken to hospitals. The number of fatalities was not immediately known.
"The main focus is to rescue people." said Miami-Dade Police Director Juan Perez. "As soon as those efforts are over, our homicide bureau will take the lead."
The 950-ton, 174-foot span was assembled by the side of the highway and moved into place Saturday to great fanfare. The $14.2 million bridge connected Florida International University and the city of Sweetwater. It was expected to open to foot traffic next year.
"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," the school said in a statement.
The National Transportation Safety Board sent investigators to the scene. Gov. Rick Scott said he was headed there as well.
"We have a national tragedy on our hands," Sweetwater Mayor Orlando Lopez said.
The "accelerated bridge construction" method was supposed to reduce risks to workers and pedestrians and minimize traffic disruption, the university said.
"FIU is about building bridges and student safety. This project accomplishes our mission beautifully," FIU President Mark B. Rosenberg said in the statement Saturday.
First-of-its-kind pedestrian bridge “swings” into place. “FIU is about building bridges and student safety. This project accomplishes our mission beautifully,” -President Mark B. Rosenberg. https://t.co/x8gPM9A4DG #worldsahead pic.twitter.com/mPEMeh2zmw— FIU (@FIU) March 10, 2018
Cristina Rodriguez, a 23-year-old junior who was on spring break with other students, said she was not surprised when she heard the bridge collapsed.
"I just felt the bridge was done too quickly to believe the bridge was stable and sound to support everything that was on there," said Rodriguez, who was not on campus Thursday but drives through the intersection almost daily.
MCM, the Miami-based construction management firm that won the bridge contract, took its website down on Thursday. But an archived version of the website featured a news release touting the project.
"This our first Design-Build with FIGG Bridge Engineers, a nationally acclaimed, award-winning firm based out of Tallahassee. FIGG has designed iconic bridges all over the country, including Boston's famous Leonard P. Zakim Bridge and Florida's Sunshine Skyway Bridge."
RELATED: Collapsed Florida International University bridge was just installed days ago
MCM said on twitter that it was "a family business and we are all devastated and doing everything we can to assist. We will conduct a full investigation to determine exactly what went wrong and will cooperate with investigators on scene in every way."
FIGG said in a statement it was "stunned by today's tragic collapse."
"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."
White House press secretary Sarah Huckabee Sanders said the president was monitoring the situation and would offer whatever support was needed.
Rep. Mario Diaz-Balart, who spoke at a ceremony celebrating the bridge's construction over the weekend, told CBS there were going to be a lot of questions that have to be answered about what happened.
"Right now the most important thing is going to be to save people who are hopefully still alive," he said.
Florida International University is the second-largest university in the state, with 55,000 students. Most of its students live off-campus. The bridge was supposed to be a safe way to cross a busy street and a plaza-like public space with seating where people could gather.
In August 2017, a university student was killed crossing the road that the bridge was supposed to span.
Florida International University is also home to the National Hurricane Center.
I have spoken with Miami-Dade County Police Chief Juan Perez about the pedestrian bridge collapse at FIU. I will be in constant communication with law enforcement throughout the day.— Rick Scott (@FLGovScott) March 15, 2018
I’m on my way to Florida International University to be briefed by local law enforcement and university officials. pic.twitter.com/4RyoeELh9m— Rick Scott (@FLGovScott) March 15, 2018
Associated Press writers Kelli Kennedy in Fort Lauderdale, Curt Anderson in Miami and Tamara Lush in St. Petersburg contributed to this report.