Sarah, we have answered the cause of the mysterious explosion last night. From the cops:

Last night at approximately 8:05 p.m., 911 calls were received about a loud explosion being heard near the Sellwood Bridge. Portland Police Officers and Fire Bureau personnel responded to the area but were unable to locate what caused the explosion. This morning, investigators from the Portland Police Bureau's Explosives Disposal Unit, (EDU), went to the area after reviewing information from citizen's calls to 911 and from local blogging information.

Investigators did locate evidence of a large pipe bomb this morning in Powers Marine Park that appears to have been detonated. Investigators believe that the sound was amplified in an easterly direction because of its placement near the bank and river and due to the low cloud cover. Police do not have any evidence at this time that anyone was injured during the explosion or that any property damage occur ed.

This is an ongoing investigation and EDU officers will continue to investigate this incident.