By Madison Park and Joe Sutton, CNN

(CNN) -- A recovery effort was underway early Friday morning in Miami after a pedestrian bridge collapsed onto a road near Florida International University, crushing cars and killing at least six people.

After searching through the night for survivors, emergency crews shifted their focus from the initial rescue mission, officials said during a pre-dawn news conference. Their work included the "very slow process" of digging through rubble to preserve evidence and ensure public safety around the unstable bridge remnants, Miami-Dade Police spokesman Alvaro Zabaleta said.

At least nine people were taken to hospitals, authorities said, after the bridge failure Thursday afternoon that one witness said "sounded like the world was ending."

The structure's 950-ton main span had just been installed Saturday over eight lanes as part of a project aimed at making the area safer for students. It was built using an accelerated construction process and designed to withstand a Category 5 hurricane.

Latest developments

• The identities of the six deceased victims were not immediately released as authorities worked to contact family members, Zabaleta said.

• Florida Sen. Marco Rubio, who visited the bridge site Thursday, posted 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." Rubio has been a visiting professor at the university for the past 10 years.

• Florida Gov. Rick Scott said state officials will determine whether any wrongdoing led to the collapse and, if so, he vowed that people would be held accountable.

• The university's president, Mark Rosenberg, said FIU followed required processes during the bridge's construction and all contractors were certified by the state.

Witness: 'Sounded like the world was ending'

Witnesses described hearing a loud boom Thursday and moments later, finding victims, including construction workers, strewn through the wreckage. Other people were trapped in cars, they said.

Giovanni Hernandez said it "sounded like a bomb, like multiple bombs in one."

"It sounded like the world was ending, and when you look back, all you see is the bridge on the floor. It was awful," he told CNN affiliate WSVN.

Sgt. Jenna Mendez of the Sweetwater, Florida Police Department was stopped at a red light one intersection away when she saw the bridge crumple. At first, she though, "Why are they taking the bridge down?" she told CNN affiliate WINK.

After watching the collapse, Mendez rushed into the debris and found four injured construction workers.

"They were working on top," Mendez told CNN affiliate WPLG. "They fell several feet, obviously. We had one that wasn't breathing. We had one that had laceration to his head that was unconscious, and we had two that were in major shock."

Doctors and medical students ran to the scene from a nearby building and started treating victims, said Isabella Carrasco, a student at the University of Miami who had just passed under the bridge in a car.

Carrasco saw at least five cars crushed beneath the bridge, she said.

"Someone on the side of the road had asked a police officer if she had heard any response from the people inside the car," Carrasco said, "and she shook her head and said no."

Kendall Regional Medical Center received 10 patients, including two in critical condition, said Dr. Mark G. McKenney, the trauma medical director.

Bridge designed to withstand a Category 5 hurricane

The bridge, which had been under construction, was meant to connect the FIU campus to the Sweetwater neighborhood, home to more than 4,000 of its students, according to a news release on the school's website. FIU is the second largest public university in Florida, with about 50,000 students.

The bridge was scheduled to open to foot traffic and cyclists in 2019, and was designed boost safety on a busy street where an FIU student was fatally struck last year by a vehicle.

"It is 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," Rosenberg said in a video.

"The bridge was about collaboration, about neighborliness, about doing the right thing," he said. "But today, we're sad. And all we can do is promise a very thorough investigation, to getting to the bottom of this and mourn those who we have lost."

The bridge was designed to withstand the strength of a Category 5 hurricane, according to a fact sheet on FIU's website, and it was supposed to last for more than 100 years. It cost $14.2 million to build and was funded through a $19.4 million grant from the US Department of Transportation.

Accelerated Bridge Construction (ABC) technology was used in the construction, according to the university. ABC streamlines the building process so bridge construction projects can be completed quicker and be more cost-efficient.

Construction firms vow to investigate

Companies involved in the bridge construction expressed sympathy for the victims and pledged to find out what happened.

MCM, one of the companies, said in a statement it would "conduct a full investigation to determine exactly what went wrong and will cooperate with investigators on scene in every way."

Another company, FIGG Bridge Engineers, which designed the pedestrian bridge stated: "We will fully cooperate with every appropriate authority in reviewing what happened and why. In our 40-year history, nothing like this has ever happened before."

Bolton Perez and Associates, a third company involved in the construction of the bridge, is not commenting at this time.

The National Transportation Safety Board will lead the federal inquiry, and a team of investigators arrived Thursday night.

CORRECTION: This story has been updated to correct the number of people killed.