In photos: Explosion rocks downtown Nashville

An explosion in downtown Nashville on Christmas Day damaged dozens of buildings and is believed to be an intentional act, authorities said.

Officers with the Metro Nashville Police Department were responding to a call of shots fired early Friday morning when they came upon an RV parked on 2nd Avenue North, according to the police.

The RV was playing a recorded message that indicated a bomb would explode in 15 minutes, Metro Nashville Police Chief John Drake said during a news conference Friday.

The bomb squad was responding when the RV exploded at 6:30 a.m. CT, police spokesman Don Aaron said.

There were no known credible threats in the Nashville area that would have signaled an impending attack on or before Christmas, a federal law enforcement source told CNN.