A dozen city-owned vehicles were set afire in a fenced-off lot beneath a freeway ramp Monday, and an explosion from one natural-gas powered car sent debris flying onto the road above, authorities said.

No one was hurt and no outside property was damaged in the fires, which were reported to the Seattle Fire Department at about 2:30 a.m.

Firefighters put out the blazes, and investigators determined that the fires were intentionally set, Seattle fire Lt. Harold Webb said.

The lot is surrounded by a fence topped with razor wire, and investigators did not immediately find any places where an arsonist may have cut or forced their way into the area, Seattle police spokesman Sean Whitcomb said.

Investigators were treating the fires as a case of vandalism.

"We don't know if it was one person or more than one person that is responsible," Whitcomb said. "One person was seen in the vicinity of this parking lot, and that is somebody we would certainly like to talk to."

Some of the cars in the lot were intentionally set ablaze, while others caught fire because they were parked next to torched vehicles, he said.

Among the vehicles burned in the incident were at least two police cars. The vehicles were no longer in service, Whitcomb said.