SAN DIEGO -- When a major Mexican drug cartel opened a branch office here on the California side of the border, U.S. authorities tapped into their cellphones - then listened, watched and waited.

Their surveillance effort captured more than 50,000 calls over six months, conversations that reached deep into Mexico and helped build a sprawling case against 43 suspects - including Mexican police and top officials - allegedly linked to a savage trafficking ring known as the Fernando Sanchez Organization.

According to the wiretaps and confidential informants, the suspects plotted kidnappings and killings and hired American teenage girls, with nicknames like Dopey, to smuggle quarter-pound loads of methamphetamine across the border for $100 a trip. To send a message to a rival, they dumped a disemboweled dog in his mother's front yard.

But U.S. law enforcement officials say the most worrisome thing about the Fernando Sanchez Organization was how aggressively it moved to set up operations in the United States, working out of a San Diego apartment it called "The Office."

At a time of heightened concern in Washington that drug violence along the border may spill into the United States, the case dubbed "Luz Verde," or Green Light, shows how Mexican cartels are trying to build up their U.S. presence.

The Fernando Sanchez Organization's San Diego venture functioned almost like a franchise, prosecutors say, giving it greater control over lucrative smuggling routes and drug distribution networks north of the border.

"They moved back and forth, from one side to the other. They commuted. We had lieutenants of the organization living here in San Diego and ordering kidnappings and murders in Mexico," said Todd Robinson, the assistant U.S. attorney who will prosecute the alleged drug ring next year.

The case shows that as the border becomes less of an operational barrier for Mexican cartels, it appears to be less of one for U.S. surveillance efforts. Because the suspects' cellphone and radio traffic could be captured by towers on the northern side of the border, U.S. agents were able to eavesdrop on calls made on Mexican cellphones, between two callers in Mexico - a tactic prosecutors say has never been deployed so extensively.

Captured on one wiretap: a cartel leader, a former homicide detective from Tijuana, negotiating with a Mexican state judicial police officer about a job offer to lead a death squad.

Recorded on other calls: the operation's biggest catch, Jesus Quinones Marquez, a high-ranking Mexican official and alleged cartel operative code-named "El Rinon," or "The Kidney." As he worked and socialized with U.S. law enforcement officials in his role as international liaison for the Baja California attorney general's office, Quinones passed confidential information to cartel bosses and directed Mexican police to take action against rival traffickers, prosecutors say.

He and 34 other suspects are now in U.S. jails. The remaining eight are still at large.

Investigators say it is not unusual for Mexican cartel leaders and their underlings to move north to seek refuge, or place representatives in such cities as Los Angeles, Chicago and Atlanta to manage large deliveries of drugs. But the Fernando Sanchez Organization was more ambitious. It was building a network in San Diego, complete with senior managers to facilitate large and small drug shipments and sales.