San Fransisco ? Forget, for a moment, computer viruses and sophisticated cybercrimes. A hacksaw and a few other tools were probably all it took for someone to sever eight fiber-optic cables in Silicon Valley this week, knocking out cell phone, landline and Internet service.

The attack was a reminder of the fragility of the telecommunications networks that are increasingly important in our lives. Yet physical sabotage of the networks is extremely rare, and far overshadowed by natural disasters like hurricanes. Security experts were unable to recall a similar incident.

Cables were cut early Thursday in San Jose and nearby San Carlos, wiping out telecom service to tens of thousands of homes and businesses. Some people were still able to place local calls, but 911 service disappeared.

A woman in Gilroy was forced to flee her home during a robbery because she couldn’t call 911. She rushed to a nearby firehouse to report the crime, city spokesman Joe Kline said.

Services returned later Thursday as repairs progressed.

Police in San Jose have received leads from potential witnesses, Sgt. Ronnie Lopez said, and FBI spokesman Brian Hale said the incident had no connection to terrorism. He did not elaborate on how that determination had been made.

Internet sabotage for the purpose of extortion or to silence an opponent’s Web site is common, but the tools are usually software, not from a hardware store. Thieves sometimes target phone and power lines because the copper has scrap value, but that isn’t true of optical fiber.