CIUDAD JUAREZ, Mexico -- The disappearance here earlier this year of Manuel Hernandez, a Texas drywall contractor and former marijuana dealer, with two relatives began like many others. Uniformed federal policemen smashed into a working-class home and dragged the men away, in full view of municipal police officers and frightened neighbors.

Then came the official denials. Mr. Hernandez's wife, Maria Elba, trudged for days from police stations to hospitals and morgues, everywhere getting the same brusque treatment: nobody knew anything about Mr. Hernandez. Although he is an American citizen, the United States Consulate was not much help either.

Nine months later, Mr. Hernandez is still missing. His case is part of a pattern unfolding just across America's southwest border: the disappearance of scores, perhaps hundreds, of people at the hands of Mexican security forces. Human rights groups say many of the disappearances appear to be part of a dirty war against people suspected of being drug traffickers.

In Juarez alone, a gritty border city that is a crossroads in a multibillion-dollar drug trade, nearly 90 people have vanished, including 8 United States citizens. Like Mr. Hernandez, many have had at least peripheral ties with the narcotics underworld.

The evidence in some cases suggests that the victims were arrested and killed by Mexican police officers or soldiers who were hired by traffickers to eliminate rivals or punish debtors. In other cases, the victims appear to have been detained for interrogation by anti-drug agents before they vanished.

The Mexican authorities say they are looking into the disappearances and have appointed a special investigator, but not one case has been resolved. Nowhere in the world have so many people disappeared in the context of drug-related violence, human rights groups say.

''There's just no parallel to what's happening in Mexico's northern states,'' said Morris Tidball Binz, who heads Amnesty International's programs in Latin America. ''We're seeing disappearances of the type seen in the 1970's, and the number of reported cases has shot up over the last year and a half.

''The person vanishes, and even though the police or the military are responsible, there is absolute denial from the authorities.''

To combat official stonewalling, relatives of scores of the missing have joined a newly formed Association of Relatives of Disappeared Persons. Its members include people from Juarez and El Paso, which together form one metropolitan area straddling the border. Their aim is to press the authorities for a full accounting, but they complain that officials have remained unresponsive.

Two Americans, Jaime F. Hervella, an El Paso accountant, and Saul Sanchez, a retired Labor Department employee from Laredo, Tex., founded the association after a fruitless three-year search for Mr. Sanchez's son, Saul Sanchez Jr. A 35-year-old United States Navy veteran, Mr. Sanchez Jr. disappeared in Juarez with his wife, Abigail, in May 1994 while selling microwave communications equipment to Mexico's federal police.

''The authorities treated us as if to say, 'Look, your relatives are gone, and that's just too bad,' '' said Mr. Hervella, the missing man's godfather.

This summer the two Americans placed newspaper advertisements inviting others with loved ones lost in Juarez to get in touch with the association's office, which is next to Mr. Hervella's accounting business in an El Paso shopping center.

The response has been stunning. In recent weeks, dozens of relatives, including Mrs. Hernandez, have called in to tell of the disappearance of more than 50 people in the last three years, most of them after detention by Chihuahua State or federal police.

A list of people who have disappeared in Juarez since 1993, compiled by a local newspaper, Norte, bears 56 names.

Reports of disappearances have emerged recently in other important drug marketplaces across northern Mexico as well. In the states of Baja California, Sonora and Sinaloa, families have reported that since mid-1996 about 20 relatives have vanished after detention.

But nowhere have so many cases been reported as in Ciudad Juarez, possibly because Mexico's largest drug cartel is based here. The Chihuahua authorities have compiled a list of 100 people who have disappeared in the state this year alone.

These disappearances are in addition to the scores of bodies dumped in ditches and fields around Juarez every year, most of them victims of drug or sexual violence.

The disappeared people include a wealthy onetime Government prosecutor whose wife now drives to association meetings in a Mercedes-Benz, an adviser to a congressman from Mexico's governing party, a recently retired Chihuahua state policeman, a former Mexican Army lieutenant and an assembly line worker whose grieving mother lives in a dirt-floor shack.

Opening the association's office in El Paso seems to have been crucial for its success. Many relatives say that they are terrified and that locating the association in Juarez would have been unthinkable, for fear that its offices would be attacked. But the association has registered with the Mexican Government and holds meetings in Juarez.

For its part, the Clinton Administration largely appears to have turned a blind eye toward the disappearances, consistently praising the Mexican Government's anti-drug efforts. Officials at the United States Consulate say their involvement has been limited because the families of only two of the eight missing Americans have sought help. In those two cases, family members said consular officials had acted sluggishly on their behalf.

But consular officials denied that. ''Protecting American citizens is the most important thing we do here,'' said David C. Stewart, the consulate's No. 2 official.

Who is responsible for the disappearances? ''I wish I knew,'' the Mayor of Ciudad Juarez, Juan Ramon Galindo, said in an interview. ''But I wouldn't be surprised to find that federal police are involved.

''One of our problems is that our officers can be bought. These are Mexican, not American police, and they reflect Mexico's problems: the lack of education, poverty. We can't hope that they will act like police from other places.''

Juarez is the main gateway used by the drug cartel controlled by Amado Carrillo Fuentes until his death in July. Many of the abductions appear to have been carried out by corrupt police officers on the cartel's behalf, to settle scores, punish informants or protect its turf against rivals. Other abductions appear to have been committed by Mexican security officials in overzealous anti-drug operations.

''What worries us enormously is the involvement of men with federal or state judicial police credentials or uniforms,'' said Alberto Medrano Villarreal, president of the Juarez Bar Association. ''Just because people are suspected of involvement in the drug trade does not mean you can allow them to be seized and killed without trial. Our entire system of law is being violated.''

The Bar Association published open letters in Juarez newspapers this year, including one during a recent visit by President Ernesto Zedillo, urging him to intervene.

Earlier this year, Jorge Madrazo, Mexico's Attorney General, quietly appointed a 72-year-old lawyer, Francisco Hernandez Vazquez, as a special prosecutor to investigate the disappearances.

''In the majority of cases, there are signs of the involvement of armed elements, who people believe belong to one of the police forces because they use uniforms or insignia or vehicles associated with the police,'' Mr. Hernandez Vazquez said in an interview.

But he added: ''There is nothing to prove that the disappearances reflect a policy of the Mexican state. They just appear to reflect the actions of certain police groups.''

The armed men appear to wield tremendous influence, however. No one interferes with them.

In February 1995, for example, after a Colombian man was acquitted of drug charges in a Federal trial in El Paso, American agents escorted him to the international bridge to Juarez and released him to his Mexican lawyer.

As the two men began walking toward Juarez, six men drove up, waved Mexican federal police credentials and dragged them into their car. Hours later, the federal police acknowledged to relatives of the men that they were in custody, but they were never seen again.

Agents of the National Institute to Combat Drugs, Mexico's now-defunct anti-narcotics agency, were implicated in the disappearance in November 1995 of two brothers who owned a Juarez steak house, along with an employee.

A man who washed cars in a police garage testified that two days after the three were reported missing, he saw federal agents shove the restaurant worker into a vehicle at the garage.

The Governor of Chihuahua, Francisco Barrio Terrazas, acknowledged the Government's involvement in that case. ''We've found that there were officers from the institute that carried them off, apparently without having an arrest warrant,'' Mr. Barrio said weeks after their disappearance.

In June 1996, Chihuahua police searching for a missing Juarez man found his Suburban parked outside a restaurant. Inside were several agents of the anti-drug institute -- as well as the missing man, who was lying on the floor in the rear.

It appeared that the federal agents were using him to identify the restaurant's customers. His family never received further information about his whereabouts.

In fact, no relatives have learned anything reliable about the whereabouts of their disappeared loved ones, although their fears have fueled speculation.

Several families said they believed that their relatives had been arrested and turned over to the Drug Enforcement Administration and were being held incomunicado in the United States.

James J. McGivney, a spokesman for the agency, said: ''That is an absurd claim, just nonsense.''

Mr. Hernandez Vazquez, the Government investigator, said police officers in Juarez had told him, ''The desert around Ciudad Juarez is a vast cemetery.''

And Manuel Hernandez's wife said a Government detective had told her that traffickers had murdered her husband on a distant ranch. ''He told me they use a big oven to cremate the disappeared,'' she said.