The conviction of Rafael Caro Quintero was overturned on a technicality this week, allowing Caro Quintero to leave prison on Friday.
Caro Quintero, 61, spent 28 years of a 40-year sentence in custody for the 1985 death of Enrique Camarena, a Drug Enforcement Agency agent who was kidnapped, tortured and killed by the Guadalajara drug ring Caro Quintero helped found.
"Caro Quintero was the mastermind and organizer of this atrocious act," the DEA said in a written statement. "We are reminded every day of the ultimate sacrifice paid by Special Agent Camarena."
Caro Quintero's conviction was tossed out by a three-judge panel on jurisdictional grounds. The judges ruled Camarena should not have been considered a U.S. diplomat, so his trial should have been held in municipal rather than federal court, the Los Angeles Times said.
The DEA vowed to continue pressing Mexico to extradite Caro Quintero to the United States for prosecution in the Camarena case.
Mexico's attorney general's office also expressed unhappiness at Caro Quintero's release and said it would investigate its options, Voice of America said.
The court did not absolve Caro Quintero of Camarena's slaying, the BBC noted, which left the door open for further prosecution.
The Times said the controversial ruling came weeks after the graft conviction of Raul Salinas, brother of former president Carlos Salinas, was also overturned.
Political observers in Mexico have expressed concerns the return to power of the venerable PRI party was resulting in an unraveling of anti-corruption reforms in Mexico.
The Times said Historian Lorenzo Meyer said in a magazine interview published this week the Salinas ruling was a not-so-subtle message to the Mexican people. "Listen, you drooling pack of idiots, the true political situation is this ... at the end of the day, we, the political class, understand the nature of corruption in this country, and it's not going to change," Meyer said.
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