A recreational vehicle parked in the deserted streets of downtown Nashville exploded early Christmas morning, causing widespread communications outages that took down police emergency systems and grounded holiday travel at the city’s airport.

Police were responding to a report of shots fired Friday when they encountered the RV blaring a recorded warning that a bomb would detonate in 15 minutes, Metro Nashville Police Chief John Drake said. Police evacuated nearby buildings and called in the bomb squad. The RV exploded shortly afterward, Drake said.

“This morning’s attack on our community was intended to create chaos and fear in this season of peace and hope. But Nashvillians have proven time and time again that the spirit of our city cannot be broken,” Mayor John Cooper said at a news conference after issuing a curfew for the area.

Police believe the blast was intentional but don’t yet know a motive or target, and Drake noted that officials had not received any threats before the explosion.

The chief said investigators at the scene “have found tissue that we believe could be remains, but we’ll have that examined and let you know at that time.” Police could not say whether it potentially came from someone inside the RV.

Three people taken to area hospitals for treatment were in stable condition Friday evening, Cooper said.

Surveillance video published on a Twitter account Friday that appeared to be across the street from the blast captured the warning issuing from the RV, “… if you can hear this message, evacuate now,” seconds before the explosion.

The blast sent black smoke and flames billowing from the heart of downtown Nashville’s tourist scene, an area packed with honky-tonks, restaurants and shops. Buildings shook and windows shattered streets away from the explosion near a building owned by AT&T that lies one block from the company’s office tower, a landmark in downtown.

“We do not know if that was a coincidence, or if that was the intention,” police spokesman Don Aaron said. He said earlier that some people were taken to the department’s central precinct for questioning but declined to give details.

AT&T said the affected building is the central office of a telephone exchange, with network equipment in it. The blast interrupted service, but the company declined to say how widespread outages were.

The AT&T outages site showed service issues in middle Tennessee and Kentucky. Several police agencies reported that their 911 systems were down because of the outage, including Knox County, home to Knoxville about 180 miles (290 kilometers) east of Nashville.

AT&T said that it was bringing in portable cell sites and was working with law enforcement to get access to make repairs to its equipment. The company noted that “power is essential to restoring” service.

The Federal Aviation Administration temporarily halted flights out of Nashville International Airport because of telecommunications issues associated with the explosion.

The FBI will be taking the lead in the investigation, agency spokesman Joel Siskovic said. Federal investigators from the Bureau of Alcohol, Tobacco, Firearms and Explosives were also on the scene. The FBI is the primary law enforcement agency responsible for investigating federal crimes, such as explosives violations and acts of terrorism.

A Philadelphia man staying in a nearby hotel said that when he heard the blast, he was knew it wasn’t harmless.

“We tried to rationalize it that it was an earthquake or something, but it was obvious it wasn’t an earthquake,” Joseph Fafara said.

When he went to look at the damage, police barricades had already been put in place.

Buck McCoy, who lives near the area, posted videos on Facebook that show water pouring down the ceiling of his home. Alarms blare in the background along with cries of people in distress. A fire is visible in the street outside.

McCoy said he heard gunfire 15 minutes before the explosion rocked his building, set cars in the street on fire and blew trees apart.

“All my windows, every single one of them got blown into the next room. If I had been standing there it would have been horrible,” he said.

“It felt like a bomb. It was that big,” he told The Associated Press.

President Donald Trump has been briefed, according to White House spokesperson Judd Deere. The U.S. Justice Department said Acting Attorney General Jeff Rosen was also briefed and directed all department resources be made available to help with the investigation.

Tennessee Gov. Bill Lee said on Twitter that the state would provide the resources necessary “to determine what happened and who was responsible.”

The American Red Cross of Tennessee announced that it was working with officials to open a shelter for victims.