MEXICO CITY – 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 of killing DEA agent Enrique Camarena, calling it “deeply troubling.”

Caro Quintero, 61, 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 active 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.

In June, the Treasury Department imposed sanctions against 18 people and 15 companies that allegedly moved money for Caro Quintero.

“Caro Quintero has used a network of family members and front persons to invest his fortune into ostensibly legitimate companies and real estate projects in the city of Guadalajara” said Adam Szubin, Director of Treasury’s Office of Foreign Assets Control.

Caro Quintero has spent almost all of his sentence at a prison on the outskirts of that city, Mexico’s second-largest city.