Two years ago, U.S. and Mexican officials, frustrated by corruption in Mexican law enforcement agencies, pushed the Mexican army to take the lead in fighting the drug war. Forming the backbone of the effort were new, vetted units trained by U.S. Special Forces and given helicopters for mobility.

But now the program, begun with high hopes and effusive praise from senior officials of both countries, is facing the same evil it was formed to combat. Around 80 members of the elite units have been under investigation in recent weeks on allegations that some of them took hundreds of thousands of dollars in bribes to sneak cocaine-filled suitcases and illegal aliens through the Mexico City airport on their way to the United States. Nine of these Mexican soldiers have been jailed on formal charges and five more have been detained.

On Sunday, Mexican civilian anti-drug authorities removed 40 of the troops -- all trained under the Special Forces program -- from their assignments at the airport as a result of the corruption investigation.

The episode, which has left some U.S. drug enforcement officials newly disillusioned, comes amid a rapid and widespread expansion of training of foreign armed forces by U.S. special operations troops -- an initiative that has proceeded largely without public debate or congressional oversight. In Mexico, as in much of Latin America, the operational focus is on combating the drug trade. But here, as in Colombia, U.S. training has not succeeded in stemming the corruption and human rights abuses that have plagued anti-drug operations in the past.

The Mexican units, whose leaders were given Special Forces training at Ft. Bragg, N.C., are called Airmobile Special Forces and are widely known by their Spanish acronym GAFE. The United States pays $28 million a year for the program and 252 Mexican officers were trained in its first 18 months, with another 156 officers scheduled for training by the end of fiscal 1998, according to the Pentagon. The U.S.-trained officers then train other groups in Mexico, and by now there are supposed to be 42 100-man units stationed around the country.

Candidates for the GAFEs, supposedly the cream of the Mexican army, are vetted by Mexican and U.S. officials. Those sent for training in the United States have their names checked against databases of suspected drug traffickers kept by the Drug Enforcement Administration, the CIA and the Defense Intelligence Agency. They also receive higher salaries than troops outside the units to make bribes less tempting.

The GAFE troops who worked at the Mexico City airport were trained by Mexican trainers, not directly by U.S. Special Forces. But U.S. officials said the indications of possible graft were a blow to their efforts to establish several corps of uncorruptible drug fighters on both sides of the border.

"After a while you wonder what the hell you are doing there," said one law enforcement official. "There is no one there we can trust completely. This was supposed to be the group we could trust and work with."

Of equal concern with the arrests themselves, U.S. and Mexican officials said, was the fact that the elite troops, whose mission was to be deployed around the country as combat-ready shock troops to attack drug cartels, were being broken up, seconded to other agencies and given routine duties such as patrolling the airport.

"I don't know why those troops were there. That is not what they were supposed to be doing," one Mexican official familiar with the program said of the airport arrests. "They are supposed to be the door-kickers and have the capacity to go after the drug traffickers and offer the best support available. It is a matter of concern to us they reportedly were loaned out to other agencies, and we are investigating why that is."

Another senior Mexican official acknowledged that the arrests were "worrisome because we expected them {the elite troops} to have more commitment, to be able to have more trust in them. . . . We tried to get the best people, but we are not always successful."

U.S. and Mexican law enforcement officials and congressional investigators said there were indications that a senior general in the upper ranks of the GAFE had assigned the troops to the airport in an effort to protect illegal activities.

"There is no other reasonable explanation," said one congressional staffer investigating the case. "Those are the indications we are getting."

The detention of the GAFE members comes as U.S. law enforcement officials have begun to question the units' usefulness in fighting drug trafficking. U.S. officials said the GAFEs have participated in only one arrest of a major drug trafficker -- Adan Amezcua, nabbed earlier this year.

The latest corruption charge is only one of a continuing series of disappointments in joint programs designed to improve drug-fighting efforts in Mexico, transit zone for an estimated 60 percent of the cocaine and two-thirds of the heroin entering the United States.

A June 30 report by the U.S. General Accounting Office, the government watchdog agency, found that much of the $76 million in U.S. anti-drug aid given to the Mexican military by the Department of Defense in fiscal years 1996 and 1997 was spent on aircraft and helicopters the Mexican army has been unable or unwilling to use.

And the report found that two Knox-class frigates the Mexican navy purchased from the United States for use in counter-drug missions "were not properly outfitted and are currently inoperable."

U.S. and Mexican officials said they were more concerned, however, about the corruption allegations. Law enforcement operations at Mexico City's Benito Juarez International Airport were taken over by a GAFE in April 1997. Within the last five months, nearly 20 of the approximately 80 officers and troops assigned to the airport have been arrested on charges of protecting drug shipments, assisting illegal immigrants and shepherding electronics and other high-duty imports past customs agents, according to Mexican investigators.

The most recent case, the detention of 14 soldiers on Aug. 9, began when a general who oversees the federal police in Mexico City received an anonymous letter alleging that anti-drug agents at the airport were protecting drug loads and facilitating the entry of illegal immigrants from South and Central America.

According to a Mexican investigator familiar with the case, the members of the anti-drug unit protected suitcases each containing 22 pounds of cocaine that arrived on an Avianca flight from Bogota, Colombia, every Tuesday for the past six months. Military officials reportedly were paid $2,500 for each suitcase delivery, the investigator said.

One military officer who works on the airport detail, and who agreed to be interviewed on condition of anonymity, said the anti-drug officers routinely pull suitcases containing cocaine off the luggage carriers between the point where they are unloaded from the aircraft and the point where bags are inspected by drug-sniffing dogs. After the dogs have examined the luggage cart, the source said, the officers toss the cocaine-filled suitcases onto the baggage conveyor belt.

In addition, members of the law enforcement units allegedly used their airport passes to lead illegal immigrants from international flights to the adjoining domestic terminal, bypassing immigration proceedings and allowing them to illegally board flights to cities close to the U.S. border. One Mexican investigator said members of the Mexico City military team had been assisting an average of 20 illegal immigrants a week for the past six months and were paid $500 per person for a total of about $240,000.

A senior military officer, who asked that his name not be used, defended the airport agents who work for the military unit, alleging that the only evidence of wrongdoing against the officers is "some change in their lifestyles -- the way they dress, the cars they drive -- but that's not strong enough to get them."

U.S. officials and news reports in Mexico have tied the airport GAFE to an attempt to protect two loads of cocaine totaling 1,335 pounds that arrived on two flights from Bogota on Aug. 20.

Two other members of the military unit were arrested in March on charges of attempting to protect 332 pounds of cocaine that arrived as baggage on a commercial flight from Bogota, according to the federal attorney general's office. Mexican authorities also linked two other cocaine shipments -- one of which was hidden amid religious books -- totaling 512 pounds to that investigation. Farah reported from Washington, Moore from Mexico City.