Mr. Gallegos claimed responsibility for several of the most notorious killings in Ciudad Juárez this year, including the shooting death of Lesley A. Enriquez, a worker at the United States Consulate in Ciudad Juárez who was pregnant, and her husband, Arthur H. Redelfs, an officer at the El Paso County Jail.
The couple was leaving a children's birthday party in Ciudad Juárez on March 14 to return home to El Paso, when gunmen fired on their white S.U.V. Their seven-month-old daughter, who was in the back seat, was unharmed.
The husband of another consulate worker was also killed the same afternoon, possibly in a case of mistaken identity, as he was driving a similar vehicle returning from the same party.
The police did not say why Mr. Gallegos ordered the consulate killings.
In July, Mexican authorities announced that they had arrested another gang leader, Jesús Ernesto Chávez Castillo, known as the Camel, who they said had told them he had ordered the consulate killings because the consulate had given United States visas to members of a rival gang.
But in a statement Sunday, the federal police said that after Mr. Chávez was arrested, Mr. Gallegos ordered the killing of his wife after she visited him in jail, apparently because he felt Mr. Chávez had given the authorities too much information about Los Aztecas.
Mr. Gallegos also admitted to ordering the massacre last Jan. 31 at a teenager's party in the neighborhood of Villas del Salvárcar because he thought members of a rival gang were there, the police said. Fifteen people were killed, and the episode shocked the city and forced President Felipe Calderón to acknowledge that innocent people were being caught up in the drug war's carnage.
Over the past year, the Mexican government has had several notable successes against the drug cartels, arresting or killing top leaders of the Beltrán Leyva drug trafficking organization, and a high-ranking leader of the Sinaloa cartel. Most recently, marines surrounded and killed Antonio Ezequiel Cárdenas Guillén, a top leader of the Gulf cartel in Matamoros on Nov. 5.
Since then, fighting between drug gangs along the border west of Matamoros has sent hundreds of people fleeing from their homes.
A flurry of polls released this week show that, for the first time since Mr. Calderón began his crackdown against drug cartels four years ago, a majority of the public no longer has confidence in the government's strategy.
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