(CNN) -- The pedestrian bridge under construction near Florida International University was supposed to enhance safety -- letting walkers and cyclists cross a busy eight-lane street with less worry after a vehicle last year struck and killed an FIU student.
Instead, it collapsed Thursday, months before it was to open, crushing cars on the street, killing at least six people and leaving investigators with the difficult task of trying to figure out why it happened and who might be held responsible.
"If anybody's done anything wrong, we'll hold them accountable," Florida Gov. Rick Scott said.
Emergency crews Friday morning shifted their focus from an initial rescue mission to the "very slow process" of digging through the rubble for any more bodies and preserving evidence around the unstable bridge remnants, Miami-Dade Police spokesman Alvaro Zabaleta said.
Of the six people who died, five bodies still were under the rubble Friday morning, Zabaleta said.
At least nine people were taken to hospitals, authorities said, after the bridge failure that one witness said "sounded like the world was ending."
The structure's 950-ton main span had just been installed Saturday using an accelerated construction process meant in part to reduce the time that street traffic was halted. The bridge had been designed to withstand a Category 5 hurricane.
• 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.
• 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.
Sweetwater police Sgt. Jenna Mendez was stopped at a red light one intersection away when she saw the bridge crumple. At first, she thought, "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 under-construction bridge 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 174-foot main span had been assembled on the side of the road, allowing traffic to continue. Then, on Saturday -- at the start of FIU's weeklong spring break -- a rig moved the span into position over the street as FIU engineering and architecture students and other community members gathered to watch. The process took about six hours.
A time-lapse video that FIU posted to YouTube shows some of the assembly, as well as Saturday's maneuvering of the span into place over 8th Street.
The bridge was scheduled to open to foot traffic and cyclists in 2019, and was designed boost safety on busy 8th Street, where an 18-year-old FIU student was fatally struck by a vehicle in August.
"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, the university's president, 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, a construction firm building the bridge, 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."
The bridge's designer, FIGG Bridge Engineers, 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, would not immediately comment.
The National Transportation Safety Board will lead the federal inquiry, and a team of investigators arrived Thursday night.