Two AT&T fiber-optic cables in Livermore, California, were severed Monday night. The incident is the latest in a string of attacks on the Internet’s privately run backbone, which the FBI is investigating.

AT&T is offering a $250,000 reward in connection with the latest attacks. It’s fiber-optic network is legally considered a critical piece of the nation’s Internet infrastructure and any attackers are subject to both state and federal prosecution.

“It’s a serious matter and affects public safety at large,” AT&T spokesman Jim Greer said Tuesday, USA Today reported.

Security experts say the attacks could be the work of terrorists probing the nation’s infrastructure to see how long repairs take, or it could just be the work of a disgruntled employee.

FBI officials said it’s possible the cable cutter is disguised as a utility employee. Whoever has been attacking the cables usually opens an underground vault, climbs inside and then cuts through the cables’ protective metal conduit before severing the lines themselves, USA Today reported.

The high-capacity lines carry vast amounts of data like phone calls, emails and computer transactions.

Michele Ernst, a spokeswoman for the FBI’s San Francisco field office, told USA Today the attack is “being taken very seriously” by the bureau and its law enforcement partners.

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