AT&T said both wired and wireless customers lost service, because the lines to some cellular base stations were cut off. Spokesman John Britton would not estimate how many users were affected. About 52,000 wireline customers of Verizon Communications in nearby Morgan Hill and Gilroy were cut off because Verizon also uses the AT&T fibers, said Jon Davies, a Verizon spokesman.

The Sprint fiber that was cut in San Carlos served customers of its wireline IP data service for businesses, as well as some Sprint cellular towers. Within a few hours, Sprint had restored service by rerouting traffic, spokeswoman Crystal Davis said.

Such deliberate fiber cuts are rare, according to AT&T's Britton. Most are caused by construction crews mistakenly digging in areas where fiber is laid. As San Jose and San Carlos police continued to investigate the incidents late Thursday afternoon, AT&T announced a US$100,000 reward for information leading to the arrest and conviction of the people who cut the cables.

The incidents took place just days after contracts covering nearly 100,000 AT&T workers represented by the Communications Workers of America expired. The union last week approved a strike against the carrier if it can't get contracts it considers fair in several areas of the country. have raised the question whether the vandalism was part of a union sabotage campaign, a suggestion the CWA rejected.

"We would never condone any kind of destruction like this," spokeswoman Libby Sayre said. "We didn't do it this time. We would never do it."