More than an acre burned of Willamette River trees between Milwaukie, Lake Oswego

Authorities are calling on the public for help in an arson investigation after a fire was started on Elk Rock Island last month.

A reward is now being offered for information that moves the case forward and yields an arrest.

COURTESY PHOTO: CLACKAMAS FIRE DISTRICT #1 - A fire on Elk Rock Island was quickly contained on Sept. 9, and no one was evacuated. Milwaukie police said a fire broke out on the island, which sits on the Willamette River, on the evening of Sept. 9, as wildfires across the state burned. Though resources were spread thin, Clackamas Fire worked quickly to contain it and prevent any evacuations. It burned less than 2 acres.

"I would describe it as very fast-moving," Adria Decker, who lives nearby, told KOIN 6 News. "When I first looked out, it looked like just the size of a large campfire, and then, all of a sudden, everything started lighting up."

Authorities have video from witnesses who were recording COURTESY PHOTO: MILWAUKIE POLICE - A still photo from footage recorded by witnesses to the fire on Elk Rock Island on Sept. 9. An arson investigation is ongoing.right before the fire started. It appears to show a person standing near the fire with what police described as "possibly a fuel canister." That person left the island before first responders arrived.

The access point to Elk Rock Island is still closed. Neighbors are hoping people respect the closures so the environment can be restored.

"When I say we, I mean the city, the parks, and the volunteers are going to try and do it the smart and safe way to bring it back to where it was," Decker said.

The arson investigation is ongoing. If you have any information about the fire, contact authorities.

Crime Stoppers has offered a $2,500 cash reward for information that leads to an arrest in the case. Tips can be made anonymously through the app "P3 Tips" or call 503-823-HELP (4357).

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