A recreational vehicle parked in downtown Nashville exploded Christmas morning, injuring three, damaging some buildings, and interrupting communications services. Travel was halted outside of the local airport. Authorities say the explosion is thoughts to be intentional. The FBI is investigating.

The Metro Nashville Police department said it believes the explosion to be “an intentional act.”

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 said they don’t yet know a motive or target, and Drake noted that officials had not received any threats before the explosion.

Two law enforcement officials told The Associated Press that human remains were found in the vicinity of the explosion. It was unclear how the remains were related to the explosion or whether they might belong to the person believed to be responsible or a victim. The officials could not discuss an ongoing investigation publicly and spoke to the AP on condition of anonymity.

“We have found tissue that we believe could be remains, but we’ll have that examined and let you know at that time,” Drake said, adding that police could not say whether it potentially came from someone inside the RV.

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

Police spokesman Don Aaron told The Associated Press three people were injured in the attack. They were taken to nearby hospitals for treatment. None were in critical condition.

The vehicle was parked outside 166 Second Avenue, according to Metropolitan Nashville Police Department. The explosion happened around 6:30 a.m. Friday.

Cooper warned people to stay away from the downtown area, as much of the city is blocked off by police.

Traffic in the area was restricted as a protective K-9 sweep was conducted by police.

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.

“Service for some customers in Nashville and the surrounding areas may be affected by damage to our facilities from the explosion this morning. We are in contact with law enforcement and working as quickly and safely as possible to restore service,” AT&T spokesman Jim Greer said in an emailed statement.

The AT&T outages site showed service issues in middle Tennessee and Kentucky, including Bowling Green about 65 miles (105 kilometers) north of Nashville. Several police agencies reported that their 911 systems were down because of the outage, including Murfreesboro and Knox County, home to Knoxville about 180 miles (290 kilometers) east of Nashville.

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

Before the explosion, Buck McCoy, who lives near the area, posted videos on Facebook of his home after the explosion.

Alarms blare in the background and cries of people in great distress ring in the background. A fire is visible in the street outside.

McCoy says he says he heard gun fire 15 minutes before the explosion rocked his building. McCoy said the windows of his home were entirely blown out.

“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.

“There were about four cars on fire. I don’t know if it was so hot they just caught on fire, and the trees were all blown apart,” he said.

President Donald Trump has been briefed, according to White House spokesperson Judd Deere, who said that Trump, who is spending the holidays in Florida, will continue to receive regular updates. 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 tweeted the state will “supply all of the resources needed to determine what happened and who was responsible.”

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. Investigators from the Bureau of Alcohol, Tobacco, Firearms and Explosives were also on the scene.

Federal Bureau of Investigation spokesman Joel Siskovic said the FBI will take the lead in the investigation.

Second Avenue — or Market Street, as it was known until 1903 — contains the largest concentration of Victorian commercial facades in Nashville, with many of the two- to five-story buildings date from 1870 to 1890. The Silver Dollar Saloon, now part of the Hard Rock Café, on the corner of Second Avenue and Broadway, served as an entertainment venue for riverboat workers.

Information from The Associated Press was used in this report.