Police say a teenager was injured by shrapnel from a pipe bomb that exploded inside a portable bathroom in a public park.

Petaluma police Sgt. Andrew Urton says the 18-year-old boy was walking in Lucchesi Park in Petaluma Saturday night when he heard the explosion and felt pain in his back.

The teen managed to find a police officer nearby, who administered first aid until an ambulance could transport the boy to a hospital. The victim, who has not been identified, was in stable condition.

The Sonoma County Sheriff’s Explosive Ordnance Disposal Unit is assisting in the ongoing investigation.