Vandals cut communications cables in four locations in the South San Francisco Bay area early Thursday morning, knocking out phone and Internet service to more than 50,000 people as well as thousands of businesses.

The sabotage blacked out communications to hospitals, stores, financial networks and emergency services, such as 911 calls, according to a report in the San Francisco Chronicle. The unknown vandals had to use a special tool to remove manhole covers and then climbed down to access the cables, some as thick as a silver dollar. At least a total of 500 optic fibers were cut, according to the San Francisco Chronicle report.

AT&T — whose service was most affected by the cuts — owns cables at two of the locations, the company said in a statement. The telecommunications giant offered a $100,000 bounty for information leading to the arrest and conviction of the saboteurs.

"Anyone who tampers with, destroys or disrupts the company's network or its components is in violation of federal and state laws and AT&T will assist with any prosecution to the fullest extent of the law," the company stated on Thursday.

The attack underscored that, while security and policy experts warn that cyber attacks on critical infrastructure pose a significant danger, physical vulnerability continues to be a major issue. Earlier this week, the Wall Street Journal reported that unnamed intelligence officials had stated that Chinese and Russian hackers had infiltrated the networks of several power companies. The admissions come as the Obama administration undertakes a 60-day review of its cybersecurity policies, as two senators introduce a bill that would create a top advisor to the president, and as the National Security Agency fights to become the lead agency for cyber operations.

The vandals struck as AT&T negotiates with its employees' labor union, the Communications Workers of America. The union's contract has run out, but the workers have not yet scheduled a strike. The CWA stated through a spokesperson that its leadership does not believe any union members were involved in the sabotage, according to the Chronicle.

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