The Arellano Félix gang, based in Tijuana and led by Benjamín and his baby-faced brother, Ramón, is accused of killing hundreds of people. In 1999, the head of the D.E.A., Thomas Constantine, said the gang was ''immune to any law enforcement efforts.''

Don Thornhill, a D.E.A. agent who fought the cartel, said: ''We had so many cases compromised, the corruption was so deep, you got to the point of asking yourself, 'Why pass information on? I'm going to get a source killed.' ''

Things began to change in November 1999 with a series of secret meetings between the Americans and the Mexicans. The Americans included Mr. Chavez from the D.E.A., along with Gregory Vega, then the new United States Attorney in San Diego, and his chief drug prosecutor, Gonzalo Curiel. The Mexican side included Jorge Madrazo, then the attorney general, and two top drug prosecutors, José Patiño, known as Pepe, and José Vasconcelos.

According to participants on both sides, the Mexicans looked across the table at Mr. Chavez, Mr. Vega and Mr. Curiel, all born of Mexican parents, and the spark of recognition lit a fire.

''It couldn't but help,'' Mr. Curiel said. ''We were working without the disconnect of interpreters and barriers of culture. When it comes down to it, this involves the country of our parents.'' Mr. Vega, now in private practice, said the simple fact that the meetings were conducted in Spanish ''broke the ice.''

''It was confianza,'' he said, the Spanish word for trust.

After the November 1999 meeting, the United States, in one last try at building a professional Mexican antidrug force, sharply increased the money it provided to train Mexican law enforcement personnel. It has tripled to $12 million this year.

''If you have good investigators, you'll have good investigations,'' Mr. Vasconcelos, now chief of Mexico's new organized-crime task force, said in an interview.