SAN DIEGO — When a major Mexican drug cartel opened a branch office here, U.S. authorities tapped into its cell phones – then listened, watched and waited.

Their surveillance effort captured more than 50,000 calls in 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 the group 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 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’ cell phone 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 cell phones 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 in search of refuge or to 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.

The gang is an offshoot of the Tijuana cartel, led by baby-faced Fernando Sanchez Arellano, a nephew of the once fearsome Arellano-Felix brothers who ran the Tijuana drug trade for almost 20 years before they were captured or killed. The nephew’s organization is a weaker syndicate, at war with itself and rivals, police say, and locked in a desperate struggle to maintain market share in the highly competitive billion-dollar drug corridor into California.

Unlike the cartel crews in Mexico, which are typically built on strong ties between families or friends, the San Diego franchise recruited from U.S.-based Latino street gangs. Some recruits were illegal immigrants, others U.S. citizens, according to arrest warrants. Twelve of the 43 indicted have alleged gang affiliations in San Diego. Six of the 43 are current or former Mexican law enforcement officers. Eight are women.

“You couldn’t pick these people out of a crowd,” said Leonard Miranda, a retired captain in the Chula Vista (Calif.) Police Department who worked on the investigation. “Some of them kept a very low profile. Their family members didn’t even know.”

According to the 86-page federal racketeering indictment unsealed July 23, cartel members operated stash houses, managed smuggling crews, distributed marijuana and methamphetamine, trafficked in weapons, laundered money, committed robberies and collected drug debts. When people did not pay, they were kidnapped or targeted with execution on both sides of the border.

U.S. authorities say the wiretaps allowed them to foil murder plots and stop other violent acts. The assistant special agent in charge of the San Diego FBI office, David Bowdich, said his teams stopped the execution of two Mexican police officers. The authorities also saved a cartel associate called “Sharky” who was going to be killed because he had disrespected drug lords in Tijuana.

From their apartments by the beach or cars parked at motels, the targets of the investigation talked and talked on their cell phones.

They almost always spoke in Spanish, usually in clipped code, with lots of street slang. They bought and quickly discarded the phones. Top lieutenants often employed “alineadores,” assistants who juggled a dozen phones and took messages so that the boss would not be heard on the line. Investigators say the alleged cartel members were afraid that their calls could be monitored.

And they were right. In February, the FBI secured hard-to-get “roving” wiretaps for 44 individuals that allowed investigators to track their movements via global positioning satellites.

According to U.S. law enforcement officials, the Mexican government was not involved in the investigation.

Quinones, the high-ranking Mexican official, was a close adviser to Attorney General Rommel Moreno, the top prosecutor in Mexico’s Baja California state. He was arrested July 22 when U.S. agents invited him to the San Diego Police Department to help with an investigation. It was a setup.

Quinones’ arrest has almost certainly dealt a blow to efforts at cross-border information sharing and collaboration, though officials on both sides played down the apparent betrayal. “Would you stop going to church just because of one bad priest?” Moreno said in an interview in Tijuana.