FILE - In this Dec. 20, 2013, file photo, anti-gentrification protesters temporarily block a shuttle bus full of tech workers at a Mission District public bus stop in San Francisco. Northern California authorities say multiple shuttle buses transporting Apple and Google employees had windows broken with unknown objects while traveling on a highway south of San Francisco on Tuesday, Jan. 16, 2018. Protesters in San Francisco, where tax incentives have lured 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. (Kurtis Alexander/San Francisco Chronicle via AP, File)

FILE - In this Dec. 20, 2013, file photo, anti-gentrification protesters temporarily block a shuttle bus full of tech workers at a Mission District public bus stop in San Francisco. Northern California authorities say multiple shuttle buses transporting Apple and Google employees had windows broken with unknown objects while traveling on a highway south of San Francisco on Tuesday, Jan. 16, 2018. Protesters in San Francisco, where tax incentives have lured 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. (Kurtis Alexander/San Francisco Chronicle via AP, File)

SAN FRANCISCO (AP) — Six shuttle buses transporting Apple and Google employees were deliberately targeted by vandals who broke the windows with unknown objects while traveling on a highway south of San Francisco, authorities said Thursday.

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.

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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.

The shuttles are unmarked, but 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 with profanity on them.

Montiel said investigators are also trying to determine what objects are being used in the attacks. 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.

Company officials did not respond to an email requesting comment Thursday.