A woman in a portable restroom on a ball field at the International School of Beaverton was injured Saturday by an explosion.  She suffered a cut to the back of her head that required stitches.  She was treated and released from a hospital.

The Portland Police Bureau's bomb squad was called to search the park and the school.  No other devices were found there, but a similar bomb went off in a nearby mailbox about 15 minutes later.

It's believed the devices were bottle bombs.  They're made by combining chemicals that can be purchased at a grocery store that cause a reaction which expands the bottle and eventually explodes.

Detectives are trying to develop leads on the suspect.  They will conduct more interviews of people who were at the park to ask whether anyone saw the person who put the bomb in the porta-potty.

The softball tournament that was being held at the park has been moved to Banks.