By Hollie Silverman and Eric Levenson, CNN
The computerized voice coming from the parked white motor home loudly asked people to evacuate and warned that the vehicle would explode in minutes.
Then at 6:30 a.m. local time, it did just that.
The RV’s explosion in downtown Nashville early Friday morning left at least three people injured, set several other vehicles on fire, destroyed a number of buildings on the block and knocked out wireless service in much of the region.
The how and why of the explosion remain a mystery on Saturday, but officials said they are confident the explosion was “intentional.” Still, the Christmas date, the early morning timing and the unusual warnings broadcast on loudspeaker prior to the explosion indicate that this was no attempt at mass murder.
“It was clearly done when nobody was going to be around,” Nashville Mayor John Cooper said Friday.
No bystanders died from the blast. However, investigators found tissue that they believe could be human remains near the site of the explosion, Metro Nashville Police Department Chief John Drake said.
The blast tore into at least 41 businesses on 2nd Avenue in Nashville, a street full of historic buildings. Concerned about the structural integrity of the affected buildings, the city cordoned the area off and won’t allow anyone in until Sunday afternoon.
“It will be some time before 2nd Ave is back to normal,” Cooper said.
Tennessee Gov. Bill Lee on Saturday announced he had 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 residents were killed,” the governor said.
Several local residents told CNN they woke to the sound of gunfire early Friday morning. Police said they were called to the location after a report of shots fired around 5:30 a.m.
Once on scene, police found a white RV parked in front of an AT&T transmission building at 166 2nd Avenue North. The RV was repeatedly broadcasting a message warning of an explosion set to occur in 15 minutes, police said.
“This vehicle will explode in 15 minutes,” the voice said, according to Betsy Williams, who was staying at an apartment on 2nd Avenue. After repeating that message for a minute, the voice then said the vehicle will explode in 14 minutes, and continued counting down from there.
Six uniformed police officers 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 grew closer to its end, the message from the RV changed, according to surveillance video taken from a building across the street.
“If you can hear this message, evacuate now,” the voice said at about 6:30 a.m. “If you can hear this message, evacuate now.”
The vehicle then exploded in a bright flash of light.
Three civilians were hospitalized and are in stable condition, officials said. The force of the explosion knocked down one officer, police spokesman Don Aaron said, and caused hearing loss in another. But no officers suffered serious injuries.
Officials have no information about whether anyone was inside the RV when it exploded. Police also tweeted a photo of the RV 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 former deputy director of the FBI Andrew McCabe, a CNN law enforcement analyst. Essential pieces of evidence, however, could be scattered across a large scene, given the size of the blast.
The most important question to answer at this time is who was behind the blast, McCabe said.
“We need to know who put this thing there and detonated it,” he said. “It’s only after you figure that out that you start to talk about things like motive or purpose.”
“Everything just, I mean, everything shook. It was quite the blast,” she told CNN’s Anderson Cooper.
Three cars were set afire. Trees were down. Bricks and glass were everywhere. Another resident said the scene of destruction was like an apocalyptic movie.
AT&T spokesman Jim Greer told CNN that the company’s network hub in the city was damaged in the explosion and 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 the outages caused by the explosion. Teams on site had faced challenges, however, including having to evacuate the building due to a fire overnight.
The street is on the edge of Nashville’s hospitality and tourist district in a historic part of town.
Technical experts from the FBI lab and evidence response teams have been brought in from around the country to “help process this massive crime scene,” said FBI Special Agent in Charge Matt Foster.
ATF Special Agent in Charge Mickey French said his agency had activated its national response teams and was working alongside the FBI and Nashville police. The agency has explosive specialists, chemists and engineers involved in the evidence recovery process.
CNN’s Paul P. Murphy, Dakin Andone, Steve Almasy contributed to this report.
This article originally appeared on CNN.