Five men and one woman in a heavily guarded downtown office building guard dangerous secrets. Talking about them could get them fired or even killed. So in isolation, the elite government lawyers work toward the same goal: dismantling the Arellano Felix drug cartel.

"No one knows what the others are working on. Only I see all the information," said Jose Vasconcelos, Mexico's chief anti-drug prosecutor, who lives behind a wall of bodyguards. "That helps prevent leaks."

In an interview, Vasconcelos described how Mexico has changed the way it fights drug trafficking by creating isolated cells of prosecutors forbidden from talking to one another. He also offered new details about the capture of Benjamin Arellano Felix, a drug lord with a $2 million U.S. bounty on his head, and of the changing state of Mexico's largest smuggling cartel.

Even a few years ago, the Arellano Felixes and other drug kingpins were able to move around Mexico with relative impunity, tipped off to arrest plans by government informants. U.S. officials said the cartel has had hundreds of officials on its payroll. But these days, authorities have made an important arrest every month. More than a dozen major traffickers have been captured in the past year by Vasconcelos's group working alongside an equally secret military intelligence unit.

Vasconcelos's secret six are the prime investigators for the nation's top drug cases. Unlike in the United States, prosecutors here not only bring cases to court, but also carry out the prior detective work. Vasconcelos said that reorganizing the anti-drug task force into independent cells has cut down on leaks. Each group pursues a separate part of the investigation, such as Colombian cocaine suppliers or money-laundering operations.

Isolating the six chief investigators also controls the damage if an investigator is forced to talk -- or is bribed into doing so. Since the mid-1980s, assassins for the Arellano Felix cartel have killed dozens of prosecutors, judges and police officers, often after torturing them.

In total, 47 prosecutors are on Vasconcelos's task force, all vetted, polygraphed and psychologically tested. Eighteen are women. According to Vasconcelos, female investigators are proving especially effective "and more discreet."

The boss of Vasconcelos's counterpart unit in the military has never been publicly identified. Vasconcelos, who is in constant contact with him, referred to him as "my mirror." He said the military investigators are likewise divided into secretive cells.

U.S. officials said the military anti-drug unit has received training from the Defense Department and CIA. The unit's agents were tailing Benjamin Arellano Felix's wife and children in the days before his arrest.

Vasconcelos said the cartel is undergoing a leadership change after the arrest of Arellano Felix in March and the shooting death in February of his brother, Ramon Arellano Felix, the cartel triggerman. With those two out of the picture, law enforcement officials on both sides of the border predicted that the cartel was finished, or nearly so. But Vasconcelos said it continues to operate.

Now, he said, his team is setting its sights on two lesser-known Arellano Felix brothers, Eduardo and Javier. Vasconcelos said that Eduardo is a physician and Javier has a college degree, making them unusually well-educated traffickers.

He said it was still unclear whether any one person has taken control of the cartel. "There is no one I can point to and say for sure: 'He's the one,' " Vasconcelos said. But, he said, the cartel continues to ship cocaine, heroin and other drugs into its distribution territory, primarily California, Illinois and other Midwestern states.

Vasconcelos said his team is also looking for Manuel Aguirre Galindo, known as "The Horse" because of the shape of his face, and as "The Promote r" because he developed many of the cartel's ties with Colombian cocaine suppliers.

Donald J. Thornhill Jr., a U.S. Drug Enforcement Administration agent in San Diego, said it remained to be seen whether the new cartel leadership "has the juice that Benjamin and Ramon had."

The "cold, psychotic killings" Ramon performed -- said to number in the hundreds -- were a key reason the cartel flourished, Thornhill said. The cartel is believed responsible for as much as one-third of the cocaine consumed in the United States, either by directly supplying it or by charging other traffickers for passage through Mexico and into the United States, he said.

In all, there were seven Arellano Felix brothers. With one dead, two in prison and two being sought, two brothers remain who have never been publicly linked to the family business.

The DEA is printing new "wanted" posters about the cartel that include pictures of Eduardo and Javier Arellano Felix. "Coming soon to a border crossing near you," he said.

Vasconcelos said that cooperation with the DEA and other U.S. agencies has improved and helps crack the drug cartels. But he said the most important factors have been closer cooperation with the military investigators and keeping his team uncorrupted.

Vasconcelos meditates every day to keep focused. At times, he burns Japanese incense in his office, a bunker in the heart of downtown Mexico City. A religious man with a calm demeanor, he spoke philosophically about his work: "Each cartel is like a person -- each has its own personality."

In the end, he said, family was the chink in Benjamin Arellano Felix's armor. Vasconcelos said his team got a key break two years ago, when a white Volkswagen Jetta was spotted at a homicide scene in the northern city of Monterrey. Investigators started watching the car, and when it turned up again at a suspected cartel house in Monterrey, they found Arellano Felix family videos inside.

One of the daughters, seen on the videos, has a pronounced facial deformity. So investigators pored over school yearbooks in Monterrey until they spotted the girl, registered under a false name, Vasconcelos said. They traced her to her Monterrey address, but by the time they got there, the family had made another of its frequent moves.

Vasconcelos said they kept up discreet surveillance on the white Jetta and its owner, who turned out to be Benjamin Arellano Felix's young personal assistant, hoping that it might eventually lead them to the family. Earlier this year, the DEA passed on information about money couriers traveling from Tijuana to Mexico City. Mexican investigators, following that tip and their own intelligence, spotted the white Jetta at the Mexico City airport and followed it to Puebla, 70 miles east of the capital.

At about the same time, investigators from the military anti-drug unit spotted Benjamin Arellano Felix's wife at a city office in Puebla, paying property taxes. She led investigators to the same house where the white Jetta had led other agents.

Vasconcelos said investigators then set up surveillance on the house. He said they were careful not to watch the house 24 hours a day, as they could have been detected by Arellano Felix's security men. So he said investigators dressed as pizza delivery men, street sweepers, vendors pushing carts and in other disguises, keeping watch on the house as closely as possible.

Then one day, Vasconcelos said, they noticed a sharp increase in the security around the house: Suddenly, men appeared on nearby bridges, carrying cell phones and radios, but no weapons.

Vasconcelos said a decision was made quickly to raid the house even through the government was not sure its target was inside. Elite soldiers stormed the house at 1 a.m. on March 9 and found Benjamin Arellano Felix. Vasconcelos said he raised a gun initially, but put it down quickly in the face of overwhelming force.

Shortly after the arrest, Vasconcelos met the "smart, but very, very cold" man he had been tracking for five years, and had finally checkmated. "I told him he had been lucky before," Vasconcelos said, "but that this time he lost."

Members of the Arellano Felix family, targeted by prosecutors, in undated photo. Back, from left: Eduardo, Luis Fernando, Isabel, Francisco, Benjamin and Alicia Maria. Front, from left: Carlos, Javier, Alicia and Enedina.