SEATTLE -- Explosive devices were used to blow up three Seattle police vehicles and a mobile command unit early Thursday at a Seattle lot, investigators said.

The FBI, along with the Bureau of Alcohol, Tobacco, Firearms and Explosives, and Seattle crime investigators, are trying to track down who torched the cars at a city maintenance and storage yard.

Police and fire units responded at 5 a.m. after the fires were reported to find police vehicles burning at the Charles Street Service Center near the interchange of Interstate 5 and I-90 in Seattle.

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A witness said he heard a loud explosion.

"The flame was like a big tanker truck going off like on a freeway, because I've seen that before, and it was just a loud boom, like a big bomb, grenade, dynamite or a big pack of firecrackers. It was pretty dramatic," said the man, identified as David.

City workers told police they saw a man coming from the fenced area where the police cars were parked, police said. Officers are searching for a man who may be wearing a backpack. Police said the arsonist had to break over a barbed wire fence.

There were reports that the fires were related to a protest over "police brutality" to be held Thursday evening, but the group holding the protest, October 22nd Coalition, denies any involvement.

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After the fire was put out, police used a bomb squad robot to investigate if anything hazardous remained in the area and an arson dog checked the crime scene for scents.

Firefighters quickly controlled the fire. Fire Department spokeswoman Helen Fitzpatrick said there were no injuries.

Seattle police won't say why the police vehicles were in the lot, but city maintenance workers said the cars routinely come in for repair work. City workers were not allowed near the buildings where the fire happened on Thursday.

Workers at a nearby car dealership said Seattle detectives spent much of the day looking through surveillance video. The employees said they didn't think the dealership's cameras pointed toward the maintenance yard.

Seattle police said the mobile command unit alone cost upwards of $500,000 and was loaded with sophisticated equipment.

Police spokesman Mark Jamieson said that in 2007, several vehicles were set on fire in the same lot.

Anyone with information regarding this incident is urged to call either 911 or the Seattle Police Arson/Bomb Squad at 206-684-8980. Those wishing to remain anonymous are encouraged to call Crime Stoppers at 1-800 222-TIPS (8477) or send a text to CRIMES (274637). Your text message should include “TIP486” to ensure proper routing. If a tip leads to an arrest and the filing of charges, callers may qualify for a cash reward of up to $1000. Calls are taken 24 hours a day.