SAN FRANCISCO (AP) — 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 last week, knocking out cell phone, landline and Internet service.

The attack was a reminder of the fragility of the telecom networks that are increasingly important in consumers’ 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 emergency call service disappeared.

A woman in Gilroy was forced to flee her home during a robbery because she couldn't call police. 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.

Lopez said that whoever cut the cables knew how to use the proper tools to remove a heavy manhole cover and slice through the thick cables, which are protected by a heavy plastic sheath.

Still, it wasn't clearly an inside job. In San Carlos, where four fiber-optic cables were severed, Police Cmdr. Rich Cinfio cautioned people not to conclude that the crime required detailed knowledge of the system's workings.

"You can get the tools needed to do this at any local hardware store," Cinfio said.

AT&T, which owns six of the severed cables, posted a $100,000 reward for information leading to an arrest, then raised that to $250,000 on Friday "as the full scope of the vandalism became more clear."

Sprint Nextel said one of its cables, which provided Internet access for large business customers, was cut, too. Sprint was able to reroute traffic over other lines within a few hours of the cut, said spokeswoman Crystal Davis.

Phone and Internet service from Verizon Communications was also disrupted for about 50,000 households, since the company uses AT&T's "long-haul" telecommunications lines in the area.

Santa Clara County spokeswoman Joy Alexiou said the sheriff's department doubled patrols in areas where people's phone service was out. County workers went door-to-door checking on vulnerable people like seniors and the disabled.

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.

Greg White, director of the Center for Infrastructure Assurance and Security at the University of San Antonio, said the location of the telecom cables is known to a fair amount of people, but no one previously had shown much interest in cutting them.

"Well, now we see that not only is it possible, at least one individual has done it," he said.

Disgruntled employees and pranksters could have motives for attacks like this one, White said.

AT&T is in contract negotiations with employees on in its landline business, which maintains the fiber-optic cables, and is seeking substantial concessions on health care costs, among other things. The union denied any involvement and said it would cooperate with investigators.

"We didn't do it," said Libby Sayre, area director for the California chapter of the Communications Workers of America, the union in negotiations with AT&T. "It's not completely inexplicable why people would be inclined to speculate. But we never condone any kind of vandalism."

The severed fiber ran in underground conduits about 10 feet (3 meters) below ground level. In other places, optical fiber runs in pipes just under the ground, or in railway embankments.

Telecom carriers could increase security in their conduits, or build more lines to provide more backups. But both solutions are expensive, White said, and the costs would be passed on to customers. His institute helps municipalities figure out threats to infrastructure and prepare appropriately.

Computer security expert Bruce Schneier said the incident shouldn't raise fears of repeat episodes. He said the vandalism was the exception that proved the rule: Telecom sabotage is not easy.

"The fact that none of us can remember this happening before shows how difficult this is," he said.

"An idiot with a backhoe" accidentally cutting a line while digging is a much more common threat than a miscreant opening a manhole, Schneier said. More common still is a windblown tree falling on a utility line. Winter storms on the northern plains regularly take out communications for hundreds of communities.

"Before you lock up your manholes, you should support your trees," Schneier said.

AP Technology Writer Peter Svensson contributed from New York