PORTLAND -- Owners of the vacant Jantzen Beach hotel destroyed Sunday in a five-alarm fire owed more than $1 million in back taxes.

Five of six buildings of the Thunderbird On The River complex, on the west side of the Interstate 5 bridge in Jantzen Beach were destroyed.

A representative with Multnomah County Assessor's office said the owners of Thunderbird Hotel LLC owed $1.123,801.02 in back taxes. The hotel has been vacant since 2005.

Officials from the federal bureau of Alcohol Tobacco and Firearms were in charge of the investigation. They said the damage estimate was around $5 million. Arson had not been ruled out but a cause would likely take at least a week to discover, investigators said.

Portland Fire Marshal John Harding said in a news conference Tuesday, the vacant building had a heavy fire load due to the mattresses and furniture stored there.

ATF Special Agent Chris Porreca said the agency was charged with investigating large, commercial fires that affect a community and 30 agents were on scene.

When ATF comes to town to assist it is just that, we come as part of a team. He said their goal was to find the cause and determine if additional investigation was necessary.

Porreca added that a lot of the wood would have to be removed and reinforced to make it safe for investigators.

Harding said the building has been a concern to the fire bureau and they have conducted training drills on the property with police. Any vacant building is a concern, he said.

He added that investigators so far were unsure where the fire started. Witnesses were urged to come forward with photos or information.

History: Thunderbird hotel

Photos: Local photographer's HDR images of hotel before blaze

The last time Portland firefighters battled a five-alarm fire was in 1999, a spokesperson said.

KGWReporters MarkHanrahan and Tim Gordon contributed to this report

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