The release of Humberto Alvarez Machain is an embarrassment to the US which masterminded his kidnap in the belief that he helped torture an agent from the drug-enforcement administration (DEA) killed in Mexico by drug traffickers in 1985.
The decision is a triumph for Mexico which condemned the trial as illegal. It follows a Supreme Court ruling that Dr Alvarez Machain could be tried in the US. Many countries condemned the doctor's abduction as a violation of international law.
Dr Alvarez Machain was seized from his surgery by gunmen in April 1990 and flown to Texas where he was arrested. The US alleged he administered injections to revive a DEA agent, Enrique Camarena, whom a drugs cartel had kidnapped and tortured. Mr Camerena was captured in revenge for a US-led raid in which dollars 5bn (pounds 3.3bn) of marijuana was burnt. But the prosecution produced no proof that the agent received injections.
The judge's decision exposes the US to the accusation that it seizes nationals from their countries without evidence to convict them. According to Dr Alvarez Machain's attorney, the two main prosecution witnesses came forward only after that event.
The tension between Mexico and the US was heightened by US allegations that Mexican officials were embroiled in the drugs trade. They were accused of obstructing US investigations and associating with drugs barons.