REDWOOD CITY, Calif. >> Six shuttle buses transporting Apple and Google employees had windows broken with unknown objects while traveling on a highway south of San Francisco, authorities said today.

Four Apple charter buses and one Google bus were attacked Tuesday during the morning and evening commutes along a 16-mile stretch of Highway 280, which connects San Francisco to Silicon Valley, said California Highway Patrol Officer Art Montiel. He said no injuries were reported.

Montiel said another Apple bus had a window smashed last week along the same stretch of highway, between State Routes 84 and 85.

It appears the buses are being targeted but it’s not clear if they are being attacked by someone on the side of the road or inside a moving vehicle, Montiel said.

While the shuttles are unmarked, the double-decker buses are ubiquitous in the San Francisco Bay Area and have become a symbol of gentrification.

Protesters in San Francisco, where tax incentives have lured Twitter, Yelp, Spotify and other tech firms, have previously targeted shuttle buses ferrying employees to and from Silicon Valley, saying technology workers were driving up rents and increasing the income divide.

During 2013 and 2014 protests, demonstrators hurled rocks at the buses, slashed their tires and taped signs on them that read “F—- Off Google.”

Montiel said investigators are also trying to determine what objects are being used in the attacks but some employees reported it could be rocks or pellet guns.

“Luckily, there haven’t been any injuries or accidents but we do take this very seriously because if the driver gets distracted, it could cause a major crash and we could have injuries,” Montiel said.

Apple is rerouting its shuttles to Highway 101 as a safety precaution, he said.

An email to the company requesting comment was not immediately returned.