Nashville authorities believe a massive explosion that occurred in downtown Nashville early Christmas morning was an "intentional act" involving a parked RV. 

News 2 reports that a police office responded to a report of a "shots fired" call and found an RV parked outside an AT&T building on Second Street just before 6 a.m. The RV was sounding a warning about a "bomb" and telling people to get away from the vehicle.

One witness told local media that the warning lasted 15 minutes and was followed by a countdown.

Responding officers evacuated the residential buildings in the area after hearing the warning. They were working to get people out when the explosion detonated.

Video from around the area indicates the explosion occurred at 6:30 a.m. local time.

One officer has reportedly suffered hearing loss, according to News2

Three other people were hospitalized with injuries, the Nashville Fire Department reports. A fire department spokesperson said none of the injuries are critical.

Multiple law enforcement agencies are on scene, including Metro Nashville Police, Tennessee Highway Patrol, FBI, ATF, and the Tennessee Bureau of Investigation. FBI and ATF are taking the lead in the investigation.

K-9 bomb detection teams are searching the area for other devices as a precaution, officials said.

Multiple buildings were damaged. Water mains were shattered. A fire department spokesperson said firefighters are checking the buildings for victims, which are not expected, and to see if the buildings themselves are sound.