A parked camper van exploded in the city of Nashville, Tennessee, US, early on Christmas morning, leaving three people injured and knocking out communications systems across the state.

According to the Police, a recreational vehicle (RV) was broadcasting a warning message to leave the area and exploded a few minutes later.

The explosion remains a mystery. According to CNN, there were gunshots first that startled residents awake in a downtown Nashville community on Christmas morning.

When people looked out their windows a little before dawn, they saw an old, white motor home parked on their street.

Then the sound of a woman's voice started emanating from the recreational vehicle, with a warning that there was a bomb that was going to go off in 15 minutes.

When the RV exploded, it shattered windows and sent flaming pieces of debris through the air, damaging several buildings.

Mayor John Cooper said the blast was caused by a "deliberate bomb."

Three civilians were hospitalized and are in stable condition, officials said.

The unusual warning that came from the RV likely saved lives. Serious injuries were also avoided due to the police officers who took the warning seriously and cleared the area.

Metro Nashville Police Department Chief John Drake on Friday evening, Dec 25, said investigators found tissue near the blast site. They will be examined, he said, to determine whether they are human remains. He could not say how close the tissue was to the site where the motor home exploded.

Watch the video of the explosion below.