MOST people will have forgotten about Rafael Caro Quintero, the drug lord who walked free from a Mexican prison on August 8th and vanished into the pre-dawn darkness. Not the United States Drug Enforcement Administration (DEA). Mr Caro, who is now 60, helped sink anti-narcotics co-operation between Mexico and the United States to an all-time low after the torture and murder 28 years ago of the DEA’s Enrique “Kiki” Camarena, for which he was convicted. His release on a technicality, 28 years into a 40-year sentence, has reopened old wounds.

According to American officials, Mr Caro and his cronies in the once-dominant Guadalajara gang targeted Mr Camarena in February 1985, partly for his role in a drug bust that cost the drug lords heavily. His body was found in a shallow grave a month after he disappeared. Later the DEA obtained a tape-recording of his hours-long torture, which added to the anger and frustration.

On the hunt for the killers, DEA agents watched in fury as a Mexican federal-police commander hugged Mr Caro instead of arresting him at Guadalajara’s airport, allowing him to board a private plane for Costa Rica. When the agents finally nabbed him there, he was accompanied by the 17-year-old niece of a politician who later governed Jalisco state, of which Guadalajara is the capital.

The killing became emblematic of the sensitivity of cross-border anti-drug efforts. Frustrated by Mexico’s failure to track down those responsible, the United States all but shut down the border for a while. Mexicans, in turn, seethed when DEA agents arranged the abduction to Los Angeles in 1990 of a Mexican doctor accused of keeping Mr Camarena alive while under torture. He was acquitted.

It was a low point. The DEA treated Mr Camarena as a fallen hero, keeping his picture up on office walls and leaking feverishly against allegedly corrupt Mexican officials. But in the 1990s, negotiations for the North American Free-Trade Agreement swept the tensions under the rug. Relations improved further during the 2006-12 administration of Felipe Calderón, as both governments teamed up to go after the far more violent crime bosses who have emerged since the demise of the Guadalajara “cartel”.

Mr Caro’s release, ordered by a Mexican tribunal on the grounds that he should have been tried in a state court, not a federal one, has once again tested the ties between both countries. Mike Vigil, a former DEA operative, says the United States received no advance warning of the release. The DEA is “livid”, he says. America has since asked Mexico to rearrest him for possible extradition.

His release seems to have caught President Enrique Peña Nieto’s administration by surprise, too, but is awkward nonetheless. Since taking office in December, Mr Peña has sought to reassure everyone that his Institutional Revolutionary Party has cleaned up since it ran the country for most of the 20th century. “Does this mean we’re going back to the way it was once upon a time?” asks James Kuykendall, Mr Camarena’s former DEA supervisor in Guadalajara. Probably not. But no doubt Mr Caro hopes so.