A former top Mexican drug cartel boss, Rafael Caro Quintero, has been released after serving 28 years for the murder of a US agent, a prison source said.

US officials on Friday expressed outrage that the 40-year prison sentence had come to an end.

"The Drug Enforcement Administration is deeply troubled to learn of the decision by a Mexican court," the US agency said in a statement.

Quintero, 60, was ordered released on August 7 by a criminal court in the western state of Jalisco, a judicial official told AFP news agency.

The court said proceedings for the 1985 murder of DEA agent Enrique Camarena should have been tried in regular rather than federal courts.

Authorities have not yet commented publicly on the drug kingpin's release.

"Caro Quintero left the prison in the morning," the prison system source, who asked to remain anonymous, told AFP.

Prosecutors did not clarify whether there were other proceedings pending against Quintero or if he would be extradited to the United States.

Quintero is wanted for charges related to Camarena's murder and drug trafficking in California.

Camarena's 1985 kidnapping, torture and death strained US-Mexico relations and gave rise to a binational crackdown on drug trafficking, according to experts.

The father of Mexico's Guadalajara drug cartel, Quintero was one of Mexico's major drug lords until his capture in 1985.