The FBI is investigating nearly a dozen successful attacks on fiber optic cables in Northern California that have caused widespread Internet outages during the past year, according to media reports.

San Francisco and suburban Sacramento suffered Internet outages Tuesday after someone cut through local high-capacity fiber optic cables in the 11th act of such vandalism since last July, USA Today reported. The cables were owned by Level 3 and Zayo, Fortune magazine reported.

FBI officials believe the attacks required expertise and could be more than vandalism, with the attackers entering underground holding bunkers and cutting through conduit sheathing, according to USA Today.

“Within the past year, cables were intentionally severed,” the FBI said in a statement, as reported by CNN. “The individuals may appear to be normal telecommunications maintenance workers or possess tools consistent with that job role.”

FBI Special Agent Greg Wuthrich told USA Today that having this many incidents within this short a time frame is concerning.

“When it affects multiple companies and cities, it does become disturbing,” he said. “We definitely need the public’s assistance.”

A similar attack in February of this year occurred in Arizona, when people from Phoenix to Flagstaff lost Internet, mobile and landline phone service because a fiber optic cable had been cut.

Such attacks reveal a key vulnerability of the Internet, experts say, at a time when many parts of daily life have become dependent on the data transmitted on the global information super highway.

“Our most critical infrastructure is basically unsecured,” Roger Entner, of Recon Analytics, told USA Today.

Ralph Descheneaux, of Network Integrity Systems in North Carolina, told USA Today that fiber optic cables at some military installations are inspected daily and Descheneaux’s company makes equipment to remotely monitor fiber optic cables for signs of tampering.

“You can spend a lot of money on encryption and fire walling, but you also need to cover the basics,” he said. “At the end of the day, if you don’t protect the actual transport mechanism, you’re always going to have a point of vulnerability.”