Mexican police recently captured the man believed to be the country's most notorious drug money launderer, detained and interrogated him for three weeks, then set him free despite a federal warrant for his arrest, according to Mexican sources. The alleged money launderer, Humberto Garcia Abrego, brother of jailed Gulf cartel kingpin Juan Garcia Abrego, had been the subject of an intense manhunt since February 1997, when he allegedly paid three federal judges almost $1 million in bribes to be freed from prison. He was never questioned about the alleged payoffs during his recent incarceration, according to an official familiar with his interrogation. Samuel Gonzalez Ruiz, chief of the Mexican attorney general's Organized Crime Unit at the time of the incident, denied in an interview that Humberto Garcia Abrego was detained and released: "It's absolutely false, it never took place." But the other Mexican official backed up his account with a copy of a document from the Organized Crime Unit written over Gonzalez's name ordering Garcia Abrego's "immediate and absolute freedom." Gonzalez, who denied writing the order, resigned Jan. 1 as head of the unit, reportedly under the pressure of superiors' dissatisfaction with his handling of several major cases. The Mexican source said Garcia Abrego was arrested Oct. 18 in the quaint colonial town of San Miguel de Allende, about three hours north of Mexico City, while police were trailing other drug suspects. He was questioned under extraordinary secrecy at a safe house in the city of Queretaro near San Miguel de Allende and released on Nov. 9 after giving investigators detailed information about money laundering in the resort town of Cancun, the source said. The disclosure of Garcia Abrego's detention and release comes when Mexico's attempts to crack down on drug trafficking and corruption within its police and legal systems are under close national and international scrutiny. President Clinton arrives in Mexico Sunday for a 24-hour visit just a week before he is required under U.S. law to determine whether Mexico should be certified as an ally in combating drug trafficking. Numerous reforms designed to strengthen the rule of law have been enacted during President Ernesto Zedillo's administration. But Garcia Abrego's release despite the pending arrest warrant for drug money laundering, the alleged failure to question him about the circumstances of his previous incarceration and the secrecy surrounding the incident underscores the challenge Mexico faces in abolishing entrenched practices that permit drug kingpins to operate with impunity. As it happens, Garcia Abrego's lengthy confinement without being charged also violated a law prohibiting the jailing of suspects without charges for more than 72 hours. "He was held illegally," said the Mexican official. "As soon as 72 hours expired, we committed a crime called kidnapping." The document relating to Garcia Abrego's release and obtained by The Washington Post is a copy of the unsigned and undated duplicate of the original order, according to the Mexican official. Although one of the most frequently cited criminal justice reforms in recent years was the enactment of a law allowing Mexican prosecutors to engage in plea bargaining with suspects who cooperate in criminal investigations, it is unclear why that new authority was not used in the case of Garcia Abrego, who was simply freed from custody after turning over valuable information. The Mexican official said Garcia Abrego's capture occurred last Oct. 18 when federal police officers who were following two drug dealers in a car in Queretaro lost the subjects, and a short time later began following an identical car. When that car arrived in San Miguel de Allende, superiors radioed an order to the officers to arrest the occupants and bring them to police headquarters in Queretaro, about an hour away. After the officers returned, the officer in charge was stunned to see that instead of the two drug dealers, his men had Humberto Garcia Abrego in tow, the source said. Garcia Abrego is best known as the brother of Juan Garcia Abrego, the head of the Gulf cartel, who was considered Mexico's most powerful drug kingpin in the early 1990s. Juan Garcia Abrego, who had a $20 billion a year business shipping about a third of all the cocaine consumed in the United States, was tried and convicted in Houston in 1996 and sentenced to 11 life terms in prison. Humberto Garcia Abrego also is renowned for the murky circumstances surrounding his release from prison in late February 1997. In that case, the Mexican government withheld news of his freedom until after Clinton approved the annual certification of Mexico, even though the order for Garcia Abrego's release had been granted days earlier. Washington protested the incident for what "appeared to be unusual circumstances related to corruption," a U.S. official said. The Mexican government never provided "a satisfactory answer" for what happened, he said. Later that year, The Washington Post reported that Mexican law enforcement authorities had obtained information that Garcia Abrego, who had served just 17 months of a five-year sentence for money laundering, allegedly had paid $1 million to three federal magistrates and a lawyer to have the conviction annulled. In an interview, one of the magistrates denied being bribed and said that prosecutors had botched the case. After his release, Mexican officials issued a warrant for his arrest on new money laundering charges. A senior official in the Mexican attorney general's office said Mexican authorities never pursued the investigation of the alleged payoffs, and the source familiar with Garcia Abrego's interrogation said he was never questioned about the alleged bribes when he was picked up four months ago. Mario Melgar Adalid, chairman of the disciplinary committee of the Federal Judiciary Council -- a relatively new panel set up to investigate allegations of judicial corruption -- said his committee was aware of the bribery allegations in connection with Garcia Abrego's release, but never investigated them. Allegations of judicial corruption are widespread in Mexico. There are numerous cases in which Mexican and U.S. police say judges accepted bribes to drop charges or release drug traffickers, but few judges have ever been prosecuted or disciplined. Judges frequently argue that Mexican prosecutors present sloppy cases with insufficient evidence. The official familiar with Garcia Abrego's recent detention said that he was interrogated for about three weeks and provided important information about money laundering in Cancun by the Juarez cartel, which is now one of Mexico's top drug mafias. Partly as a result of the information he gave, according to the official familiar with the investigation, Mexican police launched an operation in Cancun, the country's most profitable tourist resort, last November and December and seized four hotels, two restaurants, nine offices, a warehouse and four yachts, in addition to numerous other properties in Mexico City and the northern state of Tamaulipas. The seized property was valued at about $200 million, according to the attorney general's office. The source said Garcia Abrego also gave investigators information about alleged drug money laundering by Raul Salinas de Gortari, the older brother of former president Carlos Salinas. Raul Salinas was recently found guilty of murder and sentenced to 50 years in prison in connection with the 1994 death of a top politician. There have been numerous unsubstantiated reports of meetings between Raul Salinas and Juan Garcia Abrego during the 1988-94 Salinas administration. Last year, Swiss authorities seized more than $100 million in bank accounts traced to Raul Salinas, saying it was drug proceeds he had collected for acting as the chief protector of drug traffickers during his brother's presidency. Salinas has said the money was obtained from legitimate business deals. CAPTION: Humberto Garcia Abrego was the subject of an intense manhunt after allegedly bribing judges to be freed from prison in 1997. ec