Investigators have received 500 tips on the Nashville explosion

The Nashville explosion timeline of events 0:57

(CNN) -

Federal investigators in Nashville are sifting through more than 500 clues to unravel the mystery surrounding the explosion that rocked the Music City on Christmas morning.

The explosion occurred at 6:30 a.m. (local time) on Friday, after a computerized voice emanating from a parked white motor home loudly urged people to evacuate, warning that the vehicle would explode in minutes.

The explosion of the vehicle injured at least three people, set several vehicles on fire, destroyed buildings on the block and left wireless service in much of the area without service.

Authorities provided little new information at a news conference Saturday about how the investigation was progressing, but FBI Special Agent in Charge Douglas Korneski said investigators were working on the case on "several fronts."

"First, our research team is going through each stone to make sure that we know as much detail as possible to answer the question of who is responsible for this, and also to understand why they did it," he said.

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That effort involves the FBI's behavior analysis unit in Quantico, Virginia, along with approximately 250 FBI staff members who work on the scene with law enforcement partners.

"Second, our evidence response teams are committed to documenting and collecting all evidence to support the facts learned by the investigation team," he commented.

As the investigation continues, there are no signs of an active manhunt, an indication that investigators are not looking for someone who still poses a danger to the public, multiple law enforcement sources told CNN.

Among the questions investigators are trying to answer is whether the AT&T broadcast building that was damaged in the blast was the target of the blast, sources say.

When asked on Saturday if the AT&T building was a target, Korneski said: "We are looking at all possible motives."

The damage is 'shocking'

Officials have said they are confident the blast was "intentional."

However, the date of Christmas, the early morning schedule, and the unusual warnings broadcast over the loudspeakers before the blast indicate that it was not a mass murder attempt.

"It was clearly done when no one was around," Nashville Mayor John Cooper said Friday.

No bystanders were killed by the explosion.

However, investigators found tissue they believe could be human remains near the blast site, Metro Nashville Police Department Chief John Drake said.

The blast swept away at least 41 businesses on Second Avenue in Nashville, a street lined with historic buildings.

Concerned about the structural integrity of the affected buildings, the city cordoned off the area and will not allow anyone in until Sunday afternoon.

"It will be some time before Second Avenue is back to normal," Cooper wrote.

Tennessee Governor Bill Lee announced Saturday that he requested a federal emergency declaration from President Donald Trump after touring the site that morning.

"The damage is shocking and it is a miracle that no inhabitant has died," said the governor.

Shooting early in the morning before the warnings

Several local residents told CNN they woke up to the sound of gunshots early Friday morning.

Police said they were called to the scene after a shooting report around 5:30 a.m.

Once on the scene, police found a white motorhome parked in front of an AT&T broadcast building at 166 Second Ave. North.

The motorhome repeatedly broadcast a warning message of an explosion that would occur in 15 minutes, police said.

"This vehicle will explode in 15 minutes," the voice said, according to Betsy Williams, who was staying in an apartment on Second Avenue.

After repeating that message for a minute, the voice said the vehicle will explode in 14 minutes and continued the countdown from there.

Six uniformed policemen who heard the message immediately began knocking on doors and evacuating residents.

Mayor Cooper praised them as heroes and said their swift action saved lives.

As the countdown drew to a close, the VR message changed, according to surveillance video taken from a building across the street.

"If you can hear this message, evacuate now," said the voice around 6:30 am. "If you can hear this message, evacuate now."

Then the vehicle exploded in a flash of light.

Three civilians were hospitalized and are in stable condition, authorities said.

The force of the blast knocked down one officer, police spokesman Don Aaron said, and caused hearing loss in another.

But no officer was seriously injured.

Authorities have no information on whether anyone was inside the motorhome when it exploded.

Police also tweeted a photo of the motorhome as it was driven into the area at 1:22 a.m. Friday.

As the investigation continues, officials will meticulously search the area for any trace of physical evidence, according to Andrew McCabe, a CNN law enforcement analyst and former deputy director of the FBI.

However, the essential evidence could be strewn across a large scene, given the size of the explosion.

The most important question to answer at this point is who was behind the explosion, McCabe said.

"We need to know who put this thing there and detonated it," he said.

"Only after realizing that does he start talking about things like motive or purpose."

Area closed to the public

Williams, the witness, told CNN that she was in her car with her family when the motorhome exploded in a fireball.

Everything just, I mean, everything shook.

It was awesome, ”he told CNN's Anderson Cooper.

Three cars caught fire.

The trees were down.

There were bricks and glass everywhere.

Another resident described the scene of destruction as like an apocalyptic movie.

AT&T spokesman Jim Greer told CNN that the company's network center in the city was damaged in the blast and that service in the Nashville area was affected.

AT&T is the parent company of CNN.

On Saturday, the company said crews were working around the clock to restore outages caused by the blast.

However, crews on site had faced challenges, such as having to evacuate the building due to a fire overnight.

The street is on the edge of Nashville's residential and tourist district in a historic part of the city.

Technical experts from the FBI laboratory and evidence response teams from around the country have been brought in to "help process this massive crime scene," said FBI Assistant Special Agent in Charge Matt Foster.

ATF Special Agent in Charge Mickey French said his agency activated its national response teams and works closely with the FBI and Nashville police.

The agency has explosives specialists, chemists and engineers who participate in the evidence recovery process.

CNN's Steve Almasy, Paul P. Murphy, Evan Perez, and Shimon Prokupecz contributed to this report.

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