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At least three people were slightly injured in the explosion of a trailer in downtown Nashville, Tennessee.
They were taken to hospitals, said a spokesman for the rescue workers on Friday.
The police assume that the detonation was caused intentionally.
A recorded announcement from the motor home had previously warned that a bomb would explode in 15 minutes, said local police chief John Drake at a press conference.
The recording, which was later played on the local broadcaster NewsChannel5, called for minutes to evacuate the area.
The motive was unclear, the police said.
Before the explosion, there was an emergency call about shots in the center of Nashville, which is popular with tourists, said a spokesman.
The police officers who had moved in would have called the bomb experts who were still on the way when the explosion occurred.
Former FBI deputy director Andrew McCabe told CNN that the police themselves may have been the target of the attack.
The explosion occurred early in the morning on Christmas Day, with most of the buildings with offices and shops on the otherwise busy street closed.
The police cordoned off the area extensively.
The Mayor of Nashville called for people to avoid downtown.
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According to the AP news agency, a body has been found nearby, according to investigators.
It was initially unclear whether it was a victim or the person who allegedly had triggered the explosion.
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The detonation caused "significant" property damage, according to authorities.
Pictures from the local television station WKRN showed damaged houses and debris lying on the street.
The footage also showed broken windows in buildings further away from the explosion.
The incumbent US President Donald Trump had been informed of the explosion, said a White House spokesman, Judd Deere.
Mayor John Cooper said on Twitter that the exact cause of the explosion is now being investigated by police and federal forces.
"It looks like a bomb has exploded," said Cooper in a statement transmitted by WKRN.
The local disaster control spoke of an "intentional bomb incident".
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Dozens of users in social networks had taken photos and videos of black smoke over the city center after the explosion at around 6:30 a.m. on Friday morning (local time).
Initially, several sources of fire could be seen.
Reporters from broadcaster WKRN said they felt the shock of the explosion in their downtown studio.
The mayor and emergency services wanted to provide more details during the day.
With almost 700,000 inhabitants, Nashville is the capital of the state of Tennessee in the southern United States.
Because of its many live music bars and record labels, it is considered the center of country music and is also called "Music City".