SAN RAFAEL (CBS SF) -- The sound of exploding, over-inflated tires on a wheelchair caused a shelter-in-place order and partial evacuation of the Marin County Civic Center in San Rafael Thursday morning, a sheriff's lieutenant said.
The wheelchair belonged to the grandfather of a sheriff's deputy who was attending a ceremony honoring sheriff's personnel for their length of service, Lt. Doug Pittman said.
The grandfather said he had the tires of the wheelchair inflated at an auto shop before he attended the 9 a.m. ceremony in the Board of Supervisors chambers, Pittman said.
Two minutes after the ceremony ended at 9:47 a.m., one of the tires of the wheelchair in the hallway exploded, Pittman said.
"No one could discount gunfire as the source of the sound," he said.
Sheriff's deputies spilled out of the chambers and began conducting a room-to-room search of third-floor offices. Employees of the County Administrator's Office evacuated the building, Pittman said.
The second tire exploded about five minutes later. The wheelchair owner then noticed the tires had exploded and others in the hallway noticed their rubber remnants on the floor, Pittman said.
After the wheelchair owner was interviewed, "we felt comfortable that the tires were the source of the noise," Pittman said.
The sound of the explosion was amplified because of where the wheelchair was left in the corridor, he said.
An "all-clear" was issued at 10:37 a.m. The California Highway Patrol and San Rafael police also responded to the incident.
Marin County spokesman Brent Ainsworth said the nearby Venetia Valley School on North San Pedro Road was locked down as a precaution during the incident.
Ainsworth said 970 employees work at the Civic Center at 3501 Civic Center Drive in San Rafael.
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