That big boom heard Sunday night near the Sellwood Bridge that's got Facebook and Twitter all lit up could have been a pipe bomb.

Portland police said Monday afternoon that investigators from the police bureau's Explosives Disposal Unit discovered what appeared to be evidence of a large pipe bomb that had been detonated in Powers Marine Park.

The explosion's big BOOM! at about 8:05 p.m. Sunday set off car alarms and startled people across Sellwood and Southeast Portland. Dozens of people called 9-1-1 reporting the sound. Within hours, a Facebook page dedicated to the "mysterious boom (3-28-10)" was created with a link to a Google map showing a large concentration of people in the Sellwood area who reported hearing the sound.

Portland firefighters responded to the area within minutes of the sound, but were unable to find anything that could have caused the explosion.
It was the second loud boom heard in the area in two weeks. Another explosion/boom was reported March 15 near the same location.

Monday morning, however, police found what they was "evidence of a large pipe bomb." Investigators believe that the sound was amplified to the east because of its placement near the bank and river and the low cloud cover.

Police said no property was damaged by the pipe bomb.

The police bureau's explosives unit is continuing its investigation.