The surprise release of Rafael Caro Quintero, a Mexican drug lord jailed for killing a US agent, has angered Washington and increased concerns about Mexico's judicial system.
Caro Quintero was sentenced to 40 years in prison for ordering the 1985 abduction, torture and murder of US Drug Enforcement Agency agent Enrique Camarena and his Mexican pilot Alfredo Zavala.
He had 12 years left to run on his sentence, and could have faced more charges in the US.
But on Wednesday a judge in the western state of Jalisco ordered the release of Caro Quintero, 60, on a technicality. By Friday the once powerful capo had left the Puente Grande prison and vanished.
US officials suspect that Mexican agents leaked information that led to Camarena's killing.
The case plunged US-Mexican relations into a crisis, and it took decades for anti-drug agencies on both sides of the border to rebuild trust.
Ties were better under the government of President Felipe Calderon, of the conservative National Action Party (PAN), who shared criminal information with US officials and extradited scores of Mexicans to face drug charges in the United States.
But Mexico's new president, Enrique Pena Nieto from the Institutional Revolutionary Party (PRI), who took office in December, has said he wants a more independent role for Mexico in the drug war.
The PRI ran Mexico with a mix of patronage and corruption for seven decades ending in 2000, and was widely seen as turning a blind eye to the activities of drug traffickers. Pena Nieto has vowed to change that image.
The legal basis of Caro Quintero's ruling however could lead to the release of at least two of his senior henchmen in the next days, including Ernesto "Don Neto" Fonseca, whose attorney told reporters he was going to seek his client's release.
The US National Security Agency said Sunday that it was "deeply concerned" by Caro Quintero's release.
"We have seen reports that another individual connected to Camarena's killing could also be released," said NSC Spokesperson Caitlin Hayden.
"We remain as committed today in seeing Quintero and others involved in this crime face justice in the United States as we were in the immediate aftermath of 'Kiki' Camarena's murder and will work closely with the Mexican authorities on this."
The US Drug Enforcement Administration had said on Friday that it was "deeply troubled."
"Caro Quintero was the mastermind and organizer of this atrocious act," the agency said.
"We are reminded every day of the ultimate sacrifice paid by Special Agent Camarena, and DEA will vigorously continue its efforts to ensure Caro Quintero faces charges in the United States for the crimes he committed."
Caro Quintero was a leading figure in the Guadalajara Cartel, a now-defunct organization that was one of the early Mexican drug mafias that linked up with Colombian criminals to smuggle cocaine and other drugs into the United States.
Guadalajara is the main city in Jalisco state.
"It is very probable that there was corruption involved in his release," said Raul Benitez, an expert on drug trafficking at the National Autonomous University of Mexico (UNAM). "We're talking about one of the most dangerous drug traffickers here."
The Jalisco court ruled that Camarena was not a diplomat, so his murder should not have been tried in federal court but rather at the state level. It then overturned the conviction and threw out the whole case.
The Jalisco judge "completely ignored" a criterion established in March by Mexico's Supreme Court, and should have turned the case over to a state court for a new judgement, said Mexican Attorney General Jesus Murillo Karam.
However, as of Sunday there was no formal US request for Caro Quintero's arrest and extradition, a Mexican government official told AFP, speaking on condition of anonymity.
This ruling raises serious questions about Mexico's justice system, "which has several cases in the penal system with serious mistakes," said Nicolas Loza, a researcher with the Latin American Faculty of Social Sciences (Flacso).
Nor is it an isolated one: Caro Quintero was set free just days after Raul Salinas de Gortari, brother of unpopular former president Carlos Salinas de Gortari (1988-1994), was exonerated of the charge of illicit enrichment in a case involving some $17 million.
The judge handling the case ruled that prosecutors were unable to prove that Salinas, who in the 1990s was a government bureaucrat, obtained his wealth improperly.
The government is appealing, but if they lose Salinas could recover millions of dollars in assets including dozens of properties.
Salinas was the poster boy of excess and corruption in the 1990s, and he spent 10 years in prison after he was convicted of money laundering and for the 1994 murder of Jose Francisco Ruiz Massieu, a senior PRI member.
His luxurious lifestyle and millions in overseas bank accounts were front-page news for years.
However, after years of legal wrangling Salinas was acquitted and released from prison in 2005.
Salinas has always claimed he was just a shrewed businessman, and innocent of all wrongdoing.