WASHINGTON (AFP) - The United States offered a US$5 million (S$6.2 million) reward on Tuesday for the capture of a drug kingpin who was released in Mexico after serving 28 years for killing an American agent.

Rafael Caro Quintero, the leader of one of Mexico's early drug mafias, was freed in August on a legal technicality although he had 12 more years of his sentence to serve.

The US Drug Enforcement Administration (DEA) said it would offer US$5 million for information leading to the arrest or conviction of Caro Quintero, who has since gone missing.

The DEA said that Caro Quintero faced charges in California for the 1985 abduction, torture and murder of its agent Enrique Camarena.

"Caro Quintero was the organizer and mastermind of this atrocious act and his unexpected release from a Mexican prison was shocking and disturbing to law enforcement professionals on both sides of the border," DEA chief Michele Leonhart said in a statement.

Ms Leonhart said that the agency would "utilise every tool available" to "bring Caro Quintero to justice." The Mexican government was apparently caught off-guard when the court in the western state of Jalisco freed Caro Quintero, who was also convicted of killing the US agent's Mexican pilot, Alfredo Zavala.

A judge has since ordered the rearrest of the 60-year-old, who was a leading figure in the Guadalajara Cartel.

The now-defunct organisation was one of the early Mexican drug mafias that linked up with Colombian groups to smuggle cocaine and other drugs into the United States.