The explosion that rocked downtown Nashville on Christmas morning, which injured three people and damaged dozens of buildings is believed to be intentional, authorities say.

A motor home was found parked in front of an AT&T transmission building around 5:30am by Metro Nashville Police Department officers while responding to a call.

At a news conference on Friday, Metro Nashville Police Chief John Drake said that a recorded message that indicated a bomb would explode in 15 minutes was heard coming from the RV.

“There were a number of people who did evacuate and then we know of some people, it didn’t go off when the message said it would and so people started coming back in, and then it went off,” said Nashville Vice Mayor Jim Shulman.

Police spokesperson Don Aaron said that the RV exploded at 6:30 am, while the bomb squad was en route.

Prior to the explosion, officers had gone door-to-door to inform residents of the situation.

“We think lives were saved by those officers doing just that,” Aaron said.

The force of the explosion knocked down one officer, and caused hearing loss in another.

Federal law enforcement sources said that there were no known credible threats in the Nashville area that would have signaled the explosion.

Another source said that the authorities are not aware of any increased chatter by known extremist groups.

It is still unclear if a person was inside the RV when it exploded, and FBI is taking lead on the investigation.

“Acting Attorney General Jeff Rosen was briefed on the incident early this morning and directed that all DOJ resources be made available to assist in the investigation,” a Justice Department spokesman said in a statement.

Tennessee Governor Bill Lee tweeted that the state would “supply all the resources needed” to help determine the cause of the explosion.