MEXICO CITY — A Mexican court on Friday ordered the release of infamous drug lord Rafael Caro Quintero after 28 years in prison, overturning his conviction for the 1985 kidnap and killing of a U.S. Drug Enforcement Administration agent. The brutal murder marked a low point in U.S.-Mexico relations.

The court threw out Caro Quintero’s 40-year sentence for the murder of Enrique Camarena, ruling he was improperly tried in a federal court for a crime that should have been treated as a state offense.

A court official who was not authorized to speak on the record said that Caro Quintero would be released because he had already served his time on other charges.

The 61-year-old Caro Quintero is considered the grandfather of Mexican drug trafficking.He established 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.

Mexico’s relations with Washington were 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.

The raid netted up to five tons of marijuana and cost Caro Quintero and his colleagues an estimated $8 billion in lost sales.

Camarena was kidnapped on Feb. 7, 1985, in Guadalajara, the capital of Jalisco state and a major drug trafficking center. 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.

At one point, U.S. Customs agents almost blocked the U.S. border with Mexico, slowing incoming traffic to a standstill while conducting searches of all Mexicans trying to enter the United States.

Camarena’s fellow U.S. Drug Enforcement Administration agents consider 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 named after him.

Caro Quintero is said to have pioneered links between Colombian cocaine cartels and the Mexican smugglers who transport their drugs into the United States.

The U.S. Embassy in Mexico City had no immediately comment on the court decision.