According to this USA Today video, the FBI is investigating a string of attacks on California backbone. The backbone is the infrastructure for all internet services.
USA Today reports there have been at least 11 attacks on Bay Area backbone connections; including the suspected attack yesterday in San Francisco.
Attackers have managed to gain access to underground facilities and fiber optic technologies. Using special equipment, attackers have severed fiber optic lines causing internet outages throughout the bay area and north.