PORTLAND, Ore. (AP) — Authorities are investigating an early morning explosion among a group of food carts in downtown Portland. The Portland Police Bureau said the blast reported at about 3:40 a.m. Sunday shattered a large number of windows in the surrounding block. Fire officials say a propane tank appears to be the source of the blast. Officers searched the area but found no one with any injuries. There was no immediate word on a potential cause for the explosion. Area streets were closed as a bomb squad swept the area for potential explosive devices.