A motor home parked on a street exploded in downtown Nashville on Friday, moments after a recorded message from the vehicle warned of a bomb.
Three people were injured in the Christmas morning blast in the heart of America's country music capital, though none critically.
Police have called it an "intentional act".
Authorities said it was uncertain whether anyone was inside the recreational vehicle when it exploded, but police reported hours later that investigators had found possible human remains nearby.
Nashville Mayor John Cooper ordered a curfew imposed around the blast site through the holiday weekend as FBI investigators comb the scene, though police said they were aware of no further threats to Tennessee's most populous city.
Officials offered no explanation as to a possible motive, and there was no claim of responsibility.
The blast came just moments after officers responded to reports of gunfire in the area and discovered the recreational vehicle parked outside an AT&T building in downtown Nashville at about 6:00 am CST (1200 GMT).
Police heard a recorded voice warning that a "bomb would detonate in 15 minutes," Nashville Police Chief John Drake told reporters.
The message, as captured in a recording broadcast later by local television news stations, said: "This area must be evacuated now. This area must be evacuated now. If you can hear this message, evacuate now. If you can hear this message, evacuate now."
Officers quickly went door-to-door in nearby buildings to hustle people to safety, and called for dispatch of the police bomb squad, which was on its way to the scene when the vehicle blew up, police spokesman Don Aaron said.
The Metro Nashville Police Department posted a photo of the recreational vehicle on its Twitter feed and said that the vehicle had arrived in the area at 1:22 am.
The fiery blast destroyed a number of other vehicles parked nearby, shattered windows and heavily damaged several adjacent buildings, sending a pall of black smoke into the sky that could be seen for miles. The mayor said a total of 41 businesses were damaged.
Fire officials said three people were taken to local hospitals with relatively minor injuries, and they were listed in stable condition. Authorities said quick action by police to begin evacuating the area before the blast likely prevented more casualties.
Shops and other buildings were closed given the early hour of the day, the Christmas holiday and prevailing COVID-19 restrictions on public gatherings, also helping to keep injuries to a minimum.
At an early evening briefing, Drake, the police chief, told reporters there were "no fatalities we know of," but said investigators were examining "tissue that we believe could be human remains" found in the vicinity of the blast.
Police said they had not ascertained whether anyone was in the motor home when it exploded.
Damage to AT&T facilities from the explosion caused widespread disruptions of telephone, internet and fiber optic TV service in central Tennessee and parts of several neighbouring states, including Mississippi, Kentucky, Alabama and Georgia, the company said.
Drake said authorities had received no threats of an attack prior to the reports of gunfire at the outset of the incident, describing the blast as a "total surprise."
"We had no pre-warnings at all," he said.
The explosion occurred about two blocks from Lower Broadway, where some of Nashville's famous live music venues are located. The Ryman Auditorium, former home of the Grand Ole Opry and just three blocks from the blast scene, was undamaged. The Gaylord Opryland and current Grand Ole Opry complexes, which sit outside the downtown area, were not impacted.
Agents from the Federal Bureau of Investigation and the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives were assisting in the probe, agency officials said.