MLK fire - Five-alarm fire in Northeast Portland

Portland, Oregon, August 8, 2013 -- Heat and bright orange flames awakened residents of Northeast Portland's Eliot neighborhood early Thursday morning as a 5-alarm fire burned and destroyed a four-story apartment under construction at the corner of Northeast Martin Luther King Jr. Blvd. and Northeast Monroe Street. Beth Nakamura/The Oregonian

Chief Erin Janssens, announcing investigators' findings on the Northeast Martin Luther King Jr. Boulevard fire at a news conference Monday.

Thursday's early-morning, five-alarm fire that

under construction on Northeast Martin Luther King Jr. Boulevard was an arson, Portland Fire Chief Erin Janssens said Monday.

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Editor's note: A more recent version of this story is now available.

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The U.S. Bureau of Alcohol Tobacco and Firearms is offering a $2,500 reward for information about the fire.

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Investigators are withholding evidence of the cause to preserve the integrity of the investigations, fire officials announced at a news conference.

Portland's dispatch center later on Monday released

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ire Lt. Rich Chatman said it was very fortunate that no one was hurt in the densely packed, residential Eliot neighborhood, which is along one of Portland's main north-south corridors.

Portland Fire & Rescue last week

to the Monroe Apartments development at about $4 million. That estimate did not include damage to neighboring homes and other property.

On Monday, the amount of damage was upgraded to $5 million, not including the surrounding properties.

No one was injured in the blaze, which caused the under-construction apartment complex to collapse.

About 135 firefighter battled the fire, and more than 1.5 million gallons of water were used. Two lanes of Northeast Martin Luther King Jr. Boulevard were closed for days afterward.

Seven federal ATF agents worked on the investigation, but Portland Fire & Rescue remains the lead agency on the case, fire officials said.

-- Stuart Tomlinson