MEXICO CITY (AP) — U.S. law enforcement officials expressed outrage over the release from prison of Mexican drug lord Rafael Caro Quintero and vowed to continue efforts to bring to justice the man who ordered the killing of a U.S. Drug Enforcement Administration agent.

Caro Quintero was sentenced to 40 years in prison for the 1985 kidnapping and killing of DEA agent Enrique Camarena but a Mexican federal court ordered his release this week saying he had been improperly tried in a federal court for state crimes.

The 60-year-old walked out of a prison in the western state of Jalisco early Friday after serving 28 years of his sentence.

The U.S. Department of Justice said it found the court's decision "deeply troubling."

"The Department of Justice, and especially the Drug Enforcement Administration, is extremely disappointed with this result," it said in a statement.

The Association of Former Federal Narcotics Agents in the United States said it was "outraged" by Caro Quintero's early release and it blamed corruption within Mexico's justice system for his early release.

"The release of this violent butcher is but another example of how good faith efforts by the U.S. to work with the Mexican government can be frustrated by those powerful dark forces that work in the shadows of the Mexican 'justice' system," the organization said in a statement.

The DEA, meanwhile, said it "will vigorously continue its efforts to ensure Caro-Quintero faces charges in the United States for the crimes he committed."

But experts say the case against Caro Quintero was flawed from the beginning and his release is the result of a stronger federal justice system in Mexico, and it's not likely to have an impact in U.S.-Mexico relations.

Caro Quintero was a founding member of one of Mexico's earliest and biggest drug cartels. He helped establish a powerful cartel based in the northwestern Mexican state of Sinaloa that later split into some of Mexico's largest cartels, including the Sinaloa and Juarez cartels.

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But he wasn't tried for drug trafficking, a federal crime in Mexico. Instead, Mexican federal prosecutors, under intense pressure from the United States, hastily put together a case against him for Camarena's kidnapping and killing, both state crimes.

"What we are seeing here is a contradiction between the need of the government to keep dangerous criminals behind bars and its respect of due process," said Raul Benitez, a security expert at Mexico's National Autonomous University.

"The United States wants Mexico to comply with due process but it is likely that due process was not followed when many criminals were caught 10 or 15 years."

Mexican courts and prosecutors have long tolerated illicit evidence such as forced confessions and have frequently based cases on questionable testimony or hearsay. Such practices have been banned by recent judicial reforms, but past cases, including those against high-level drug traffickers, are often rife with such legal violations.

Mexico's relations with Washington were badly damaged when Caro Quintero ordered Camarena kidnapped, tortured and killed, purportedly because he was angry about a raid on a 220-acre (89-hectare) marijuana plantation in central Mexico named "Rancho Bufalo" — Buffalo Ranch — that was seized by Mexican authorities at Camarena's insistence.

Camarena was kidnapped in Guadalajara, a major drug trafficking center at the time. His body and that of his Mexican pilot, both showing signs of torture, were found a month later, buried in shallow graves. American officials accused their Mexican counterparts of letting Camarena's killers get away. Caro Quintero was eventually hunted down in Costa Rica.

Times have changed since then, and cooperation has strengthened and this is likely to have little impact on the overall relationship between Mexico and the United States, said Tony Payan, an expert on U.S.-Mexico relations at the University of Texas-El Paso.

The U.S. and Mexico "will scramble for a bit, but in the end they will understand this is a very complex relationship and nothing is going to happen," Payan said. "They are not going to jeopardize the overall relationship over this."

Caro Quintero still faces charges in the United States, but Mexico's Attorney General's Office said it was unclear whether there was a current extradition request.

Attorney General Jesus Murillo Karam later said in a statement that his office is analyzing whether there are any charges pending against Caro Quintero.

The U.S. Department of Justice said it "has continued to make clear to Mexican authorities the continued interest of the United States in securing Caro Quintero's extradition so that he might face justice in the United States."

Samuel Gonzalez, Mexico's former top anti-drug prosecutor, said the U.S. government itself has been promoting, and partly financing, judicial reforms in Mexico aimed at respecting procedural guarantees for suspects, an approach Gonzalez feels has weighted the balance too far against prosecutors and victims.

"This is all thanks to the excessive focus on procedural guarantees supported by the U.S. government itself," Gonzalez said. "I warned them (U.S. officials) that they were going to get out, and they are all going to get out," he said referring to long-imprisoned drug lords such as Miguel Angel Felix Gallardo, who is also serving a sentence related to the Camarena case.