The California Highway Patrol is investigating what caused windows to shatter Tuesday on five shuttle buses carrying employees from Google and Apple along the Peninsula on Interstate 280.

Four of the buses were shuttling Apple employees, and the fifth bus was transporting Google employees, according to CHP Officer Art Montiel. The buses suffered window damage, perhaps from a BB gun or rocks, Montiel said.

All the buses had passengers on board when the windows were shattered. The technology giants run shuttle buses from San Francisco to their locations in Silicon Valley.

“We know it’s not a high-powered rifle or anything like that,” Montiel said. “The outside glass of the window broke, but the object didn’t go into the bus.”

The damage was caused on a stretch of I-280 between Highway 85 and Woodside Road, Montiel said. The buses were hit while traveling in both directions, during the morning and evening commutes, Montiel said.

The chartered buses are unmarked and do not offer any visible connection to Google or Apple, Montiel said.

“Even though they are targeting buses, we don’t believe they are targeting the specific companies,” he said.

The CHP is working with Apple on the investigation.