Vandals snipped another fiber optic cable line in the San Francisco Bay area this week, the 12th incident of its kind in the region over the past year.

The latest attack occurred in the San Joaquin Valley town of Stockton, disrupting Internet, mobile phone, and 911 service for tens of thousands of AT&T and Verizon customers in three counties east of San Francisco. Service was restored about a day after the Tuesday incident.

The FBI, which is investigating the attacks, has not stated a motive, but it said the attacks usually occur in remote areas where there are no surveillance cameras. The initial attacks on California telecommunications lines began in July 2014. Whoever is responsible appears, for the moment, to be operating with impunity.

AT&T said in a statement that the damage occurred to "1,200 feet of a fiber run that required 192 pairs of fiber to be re-fused. That's a ton of capacity. So this was a major trunk and 1,200 feet of it was damaged."

The cable is about as thick as a finger and is encased in a hard, flexible conduit. The FBI said that whoever is responsible may appear as telecom maintenance workers or "possess tools consistent with that job role."