BURIEN, Wash. -- The King County sheriff's office said the Burien apartment fire that killed three people was arson.

Sheriff’s office spokesman Sgt. John Urquhart said investigators are certain the fire was set. A $10,000 reward is offered for information leading to an arrest.

SLIDESHOW: Burien Apartment Building Destroyed By Fire

Evidence from the fire was collected to be sent to the Washington State Patrol Crime Lab Monday.

The fire early Sunday killed a boy, about 8 years old, his grandfather and another man.

The fire erupted at the Tara Apartments in the 400 block of Southwest 155th Street in Burien at 12:40 a.m.

The victims were in two separate units, one upstairs apartment and one downstairs.

At least five others were injured, including one man who is in serious condition at Harborview Medical Center.

Three buildings were involved in the fire. Investigators said that the damage estimates, including personal property, is about $1.75 million.

Demolition of the apartments is expected to begin shortly.

The Red Cross opened a shelter and is helping 15 families who have been burned out of their homes. Make A Donation or call the Red Cross at (206) 323-2345 or (360) 377-3761 to help the families in need following this fire.