MEXICO CITY (AP) - Infamous drug lord Rafael Caro Quintero walked free Friday after 28 years in prison when a court overturned his 40-year sentence for the 1985 kidnapping and killing of a U.S. Drug Enforcement Administration agent, a brutal murder that marked a low point in U.S.-Mexico relations. The U.S. Department of Justice said Friday it was extremely disappointed by the release of the man convicted in the killing of DEA agent Enrique Camarena, calling it "deeply troubling." Mexico's Attorney General Jesus Murillo Karam said in a statement that he was "worried" about the court's decision, adding that his office is analyzing whether there are any charges pending against Caro Quintero. Caro Quintero, 60, was a founding member of one of Mexico's earliest and biggest drug cartels. The court ruled Wednesday that he had been improperly tried in a federal court for a crime that should have been treated as a state offense. Prison officials were notified of the ruling on Thursday, and an official at the Jalisco state prosecutors' office said the drug lord left prison before dawn on Friday. The official was not authorized to speak on the record. News media were not alerted until hours after the release, and U.S. authorities apparently received no prior notification. "The Department of Justice and the Drug Enforcement Administration learned today that early this morning Rafael Caro Quintero was released from prison," said Justice Department spokesman Peter Carr. 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. " 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. Apparently, the U.S. had requested his extradition for the Camarena killing - something Caro Quintero can't be tried twice for - but may not have filed extradition requests for pending U.S. drug charges. 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. " Caro Quintero 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. He is still listed as one of the DEA's five top international fugitives, and U.S. authorities believe he continued to control the laundering of drug money from behind bars. "Caro Quintero continues to launder the proceeds from narcotics trafficking and he maintains an alliance with drug trafficking organizations such as the Sinaloa Cartel, most notably with Esparragoza Moreno's network," said Treasury Department spokesman John Sullivan, referring to Juan Jose Esparragoza Moreno, also known as "El Azul," or "Blue" because of the dark color of his skin, who is allegedly a top leader of the Sinaloa cartel. Caro Quintero has spent almost his entire sentence at a prison on the outskirts of that city, Mexico's second-largest city. Camarena's fellow DEA agents considered him a hero in the war against drug trafficking and the El Paso Intelligence Center, where U.S. federal agencies collect information about Mexican drug barons, is dedicated to him. Times have changed since the low point, and cooperation has strengthened, but Caro Quintero's release Friday reopened old wounds. Edward Heath, the former DEA regional director for Mexico at the time of the Camarena killing who was present during the identification of the agent's body from dental records, said the release reflected a broader lack of cooperation with the U.S. from the new Mexican government, a contrast to the policy of former President Felipe Calderon. You had a president that was working very close with our government in a quiet way. These people come in and so, boom, the curtain comes down," said Heath, now a private security consultant. It means a disrespect for our government. He said he was skeptical of the explanation that there was a justifiable legal rationale for Caro Quintero's release. "There's some collusion going on," he said. This guy is a major trafficker. This guy is bad, a mean son of a gun. The ruling left many wondering why it took so many years for judges to determine Caro Quintero was tried in the wrong court. "They were always 'political' prisoners serving sentences for as long as the U.S. kept up the pressure," said a former DEA official who once worked in Mexico. He is not authorized to talk about the case because he still does work in Mexico. The bribe money to get them out was always there. Mexican 'justice' is always built on very weak foundations. And they seem to like it that way. Sad," he added. AP Writers Michael Weissenstein and Kathy Corcoran contributed to this report.