A parked camper van exploded in the US city of Nashville, Tennessee, early on Christmas morning, injuring three people and knocking out communications systems across the state.
Possible human remains were later found near the blast site, US media report.
Police believe the powerful blast was caused deliberately.
Officers responding to reports of gunshots just before 06:00 (12:00 GMT) found a camper van broadcasting a warning message to leave the area.
The van exploded a few minutes later.
A police officer was knocked off their feet by the force of the blast, officials said.
Police have now released this image of the van – described by Nashville police as a recreational vehicle (RV) – arriving at the scene early on Friday.
The van blew up outside a building belonging to the telecoms giant AT&T, which also occupies an office tower nearby.
Buildings suffered structural damage, windows were blown out, and trees were felled. Videos posted on social media showed water from damaged pipes running down walls as alarms howled in the background.
Police emergency systems were knocked out across much of Tennessee. Flights out of Nashville International Airport were briefly halted as a result of damage done by the blast but have now resumed.
‘It felt like a bomb’
No motive has yet been established, nor do police know who was behind the incident.
A number of people have been taken to the central police precinct for questioning, a spokesman told the Associated Press.
It was not clear whether anyone was inside the vehicle at the time of the explosion, police said.
The FBI is leading the investigation. Agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives are also involved.
Resident Buck McCoy said he had been woken up by the blast. He posted a video on Facebook, showing some of the damage done, with alarms howling in the background.
“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,” Mr McCoy told AP. “It felt like a bomb. It was that big.”
The explosion hit an area of Nashville known for its restaurants and nightlife.
“To this point, we do believe that the explosion was an intentional act,” police spokesman Don Aaron told reporters.
CCTV footage posted on YouTube appeared to show the moments before the explosion, when a warning was broadcast, saying, “If you can hear this message, evacuate now”. A loud bang follows and flames and smoke fill the screen.
“It looks like a bomb went off,” Nashville Mayor John Cooper said, urging people to stay away from the area.
“This morning’s attack on our community was intended to create chaos and fear in this season of peace and hope, but the spirit of our city cannot be broken,” he said.