PETALUMA, Calif. -- An 18-year-old Petaluma man was injured when a pipe bomb exploded in a portable toilet located in a public park on the Fourth of July.

At about 10:35 p.m., Officer Matthew Capitelli was flagged down at Lucchesi Park to assist with someone who had been injured.

"The officers found... the subject had a puncture wound to his back," said Petaluma police officer Tim Lyons.

The injury was severe and the victim was losing a large amount of blood, police said.

Medical personnel arrived while Capitelli was assisting him and transported him to the hospital.

The man said he was walking by the portable bathroom located near the park when he heard a loud bang and felt the injury to his back. The portable bathroom had evidence that a pipe bomb had been detonated inside, according to police.

The shrapnel from the pipe bomb had blown through all the walls and the roof of the portable bathroom. A piece of shrapnel from the pipe bomb hit and injured the victim.

"We don't have any witnesses to the crime. No one reported it or called in any loud explosions," said Lyons. "The female that flagged down the officers just found the injured subject."

The man is in stable condition and an investigation is ongoing, according to the Police Department.