The Miami-Dade Police Department is conducting a homicide investigation into the Florida International University pedestrian bridge collapse on Thursday that killed at least six people, one of them being a student at FIU, officials said during a news conference Friday morning. 

During a 10 a.m. news conference, Juan Perez, Director of Miami-Dade Police Department, said they are working with several agencies to determine the cause of the bridge collapse and to bring closure to the families of the victims. 

Perez said the police department is calling it a homicide because people died, not because there are any criminal charges pending.

"Please do not jump to any conclusions. This is a homicide investigation, that's all it is. That means that someone died. That is it. That does not mean that there are criminal charges looming or pending, or anything like that. Is there a possibility like that? There is always a possibility for something like that to occur," he said.  

One question asked during the conference is why traffic was not shut down while workers were tightening the bridge cables and Perez said that is another thing the department is looking in to. 

The Miami-Dade Fire Rescue is working closely with the police department to help find any additional survivors and to recover the bodies of the victims. 

"This has turned into a rescue to a recovery operation," said Chief Dave Downey with the Miami-Dade Fire Rescue. 

The National Transportation Safety Board is also conducting an independent investigation, separate from the police department's investigation, to determine the cause of the accident and to help prevent it from happening again. 

Robert Sumwalt, chairman of the NTSB, said during the news conference that the investigation will be very detailed and that the agency's goal is to determine the cause and to make recommendations so that it won't happen again. 

Early Friday morning, the Miami-Dade Police Department first announced the number of people killed, saying that "at least" six people died because there might be more under the rubble. 

"There is a possibility, a sad possibility, that under the concrete there maybe additional vehicles," said Alvaro Zabaleta, a Miami-Dade Police Department detective and public information officer. 

The identities, even the ages, races and sex, of the victims have not been released, Zabaleta said, adding that nine people were sent to a local hospital and it is believed that another person drove him or herself to that hospital.

One of the dead died at the hospital, Zabaleta confirmed.

During the overnight, crews have been cleaning up the debris from the deadly pedestrian bridge collapse that has also left nine injured.

Investigators worked throughout the night at Florida International University trying to figure out what caused the bridge to collapse.

U.S. Florida Sen. Marco Rubio tweeted that the cables that suspend the bridge had loosened and the engineering firm ordered that they be tightened. He said they were tightening them when it collapsed.

Zabaleta said that engineers warned that the rescue operation has to be done carefully, not just to preserve evidence and the victims who could be under the rubble, but to also keep rescuers safe.

Engineers are concern for the rescue crews because the collapsed bridge is unstable, he said.

Witnessing the bridge falling

Many who were there described chaos as they witnessed the bridge falling.

"It sounded like a bomb. Multiple bombs in one. It was terrible. It was super loud. It sounded like the world was ending," said one witness.

Witnesses in shock, the bridge that just went up last weekend, now on the ground.

"I couldn't believe that it was falling. So I told myself, 'Why would they do this. Why are they bringing the bridge down?' I don't understand what's going on. I was in shock and when it finally hit me like, 'This is bad,'" said Sweetwater Police Department Sgt. Jenna Mendez.

Some ran towards the rubble to search for survivors.

"I was on the way to work today and just a couple blocks away from me when it happened. When I saw happened I immediately got on the radio to get the word over to the incident location so we could start helping out. So when I ran up to the scene I saw multiple workers," said Adrian Mesa, another sergeant with the Police Department.

Witnesses say they saw construction crews on the bridge when it came down.

"They were working on top and they fell several feet obviously," described Mendez.

Others on the scene say they crossed under the bridge just moments before it came crashing down.

A witness described what happened to someone he knew as the bridge fell.

"He was crossing the street at the moment that the bridge was coming down. And I don't know if he was ... I don't know really. He was just severely injured," the man said.

What the bridge meant and its history

The National Transportation Safety Board (NTSB), the FBI and Occupational Safety and Health Administration (OSHA) are already on scene preparing to start their investigation and Gov. Rick Scott has directed the Florida Highway Patrol to send help with the search and rescue efforts and traffic control.

Rubio has been an adjunct professor at FIU for the last 10 years and calls it home and it is ironic because the bridge was designed for safety. He said that there was so much pride and excitement that surrounded this project and what happened is heartbreaking.

"To see it on the ground there today, and underneath it. Those who lost their lives as a result of this and those who have been injured. It's just so tragic. We recognize that, even as we speak to you now, there are families whose hearts are being broken by the news or the thought that a loved one has perished," Rubio said.

The bridge, which was went up last weekend, cost $14.2 million and was supposed to open in 2019 as a safe way for students to cross the busy road. It was the first of its kind to be built using an accelerated bridge construction method.