Another suspect arrested shortly after the killings, Ricardo Valles de la Rosa, said the target of the shooting was Ms. Enriquez’s husband, Mr. Redelfs. Newspapers in El Paso and Juárez reported that gang leaders wanted to kill him because they said he had tormented a gang member in the El Paso County Jail.

Both Mr. Chávez and Mr. de la Rosa belong to Los Aztecas, a cross-border gang that works for La Linea. Mr. Chávez, whom the police described as a leader of the gang, was arrested with six other suspects, also believed to be members of the gang, in a raid in Ciudad Juárez on Thursday.

Mr. Chávez had served five years in prison in Louisiana on a drug charge, the police said. He was arrested in April 2008 by the Mexican Army on drug charges and later released. He also confessed that he was part of a group of gunmen that attacked a teenagers’ party at the end of January, killing 15 people. Mr. Chávez told the police that his group had mistakenly believed the people at the party were members of a rival gang, known as the Artists Assassins, or Double-A.

Andrea Simmons, a spokeswoman for the F.B.I. in El Paso, said she had no additional information about the arrest. “We’re working with Mexican authorities as part of a joint investigation,” she said.

The gun battle that killed 21 people took place about 4 a.m. Thursday about 12 miles from the Arizona border in a remote area often used for smuggling drugs and migrants. The authorities said they captured nine suspects, six of them wounded, plus eight vehicles and seven weapons.

Farther east, near the Texas border, gunmen shot at a sport utility vehicle late Wednesday night that was carrying the Chihuahua State prosecutor, Sandra Salas García, and her two bodyguards. Ms. García, 38, who was in charge of highly delicate internal investigations in Ciudad Juárez, was killed. One of the bodyguards was also killed and the other was seriously wounded.

More than 23,000 people have been killed in drug-related violence since late 2006, when President Felipe Calderón came to office vowing to stem the rising power of the drug trafficking organizations. The violence has spiked in recent months, according to tallies compiled by the Mexican news media, with the killing of a gubernatorial candidate in Tamaulipas on Monday provoking particular outrage.