Police are investigating four suspicious fires since Monday, including one that nearly killed an elderly couple, and that together caused more than $500,000 in damages.

Investigators have determined that three of the fires — two at a Kalihi tire shop and a fire that shut down the state Department of Health building — were deliberately set. A fourth fire, which sent a longtime Salt Lake couple to the hospital on Monday, also is being investigated as suspicious, police said.

Honolulu police arson detectives have interviewed witnesses in each case and are awaiting lab results from evidence gathered at the scenes. Arson is a big concern for local authorities, who saw all major criminal offenses fall last year, except for arson.

"We would ask that the public be aware of suspicious activity and if they see anything like that, to call HPD," said Honolulu Fire Capt. Emmit Kane.

The FBI defines arson as "any willful or malicious burning or attempting to burn, with or without intent to defraud, a dwelling, house, public building, motor vehicle or aircraft, or the personal property of another."

On Wednesday night, Firestone Tire and Service Center at 1414 Dillingham Blvd. was scorched by an arsonist for the second time in two weeks. The most recent fire started in a bunker filled with tires and spread to an adjoining structure.

Fire officials estimate the damage to be in excess of $220,000 and so extensive that the mauka side of the building will have to be razed. The fire followed a Jan. 22 blaze that burned a bunker filled with 200 tires, nearly gutting the inside.

Before Wednesday's fire, trash bins in and around the Firestone property have been set afire for more than three weeks between the hours of 8 and 10:30 p.m, police said. Arson investigators suspect the same people starting the trash bin fires are responsible for the Firestone blazes.

"We're aware of the situation and will be actively investigating the problem," said Kalihi police Maj. Susan Ballard.

Kane said arson investigators have completed their investigations of Monday's house fire in Salt Lake and Tuesday night's apparent fire-bombing of the main entrance to the state Department of Health building on Punchbowl Street.

Kane said both investigations have been turned over to police. Fire investigators would not discuss what was found at the scene of the Salt Lake fire.

The string of fires began at 6:10 a.m. Monday when more than 30 firefighters responded to a blaze on Ala Lehua Street in Salt Lake.

John and Nellie Takami were critically injured from smoke inhalation in the blaze that destroyed their home. The fire caused $300,000 in damage.

Tuesday night, Kane said a "deliberately set" fire damaged the front door and parts of the entryway to the DOH building at 1250 Punchbowl St. The fire caused between $10,000 and $12,000 in damage and closed the building all day Wednesday.

Firefighters found a gasoline container in the trash can near the front door of the building and said the glass door was shattered. Burn patterns from the fire indicate that a flammable liquid was used, Kane said.

Last night, an abandoned vehicle was set on fire in the Mapunapuna area and police arrested two boys, ages 13 and 14, on suspicion of second-degree criminal property damage. Detective Paul Nagata said it was too early to tell if the boys were involved in the other fires.

Nagata said police responded to a fire call about 8:30 p.m. on Kilihau Street. When the officers arrived, a witness identified two boys as being responsible for setting the fire, he said.

Reach Peter Boylan at pboylan@honoluluadvertiser.com.