You can't eat at La Langosta restaurant here anymore. A seven-foot-high brick wall was erected to block the entrance last week.

But the doors were open on the night of Jan. 30, when two Americans walked in on a private dinner hosted by the restaurant's owner, according to Mexican officials who quoted recently obtained legal testimony. It was the wrong party to interrupt. The host was Rafael Caro Quintero, now imprisoned as one of Mexico's leading narcotics traffickers, and he and his gunmen apparently mistook the Americans for agents or informers of the U.S. Drug Enforcement Administration.

John Walker, 36, a would-be novelist from Minnesota, and Alberto Radelat, 32, a would-be dentist from Fort Worth, have not been seen since. Two restaurant employes have told police that the visitors were beaten, then stabbed repeatedly by Caro Quintero and his men.

Enrique Alvarez del Castillo, governor of Jalisco State, and an aide provided a description of the restaurant employes' statements, and this description was confirmed independently.

The account of what happened to Walker and Radelat has cast new light on a series of recent disappearances of U.S. citizens in this major Mexican city. Gov. Alvarez suggested that four Jehovah's Witnesses who disappeared in December also may have been abducted by narcotics traffickers who mistakenly thought they were working for the DEA.

Caro Quintero helped plan the kidnaping of a real DEA agent, Enrique Camarena Salazar, and of his Mexican pilot in Guadalajara a week after Walker and Radelat disappeared, according to Mexican police officials. Camarena and the pilot were found dead a month later, and the killings have led to a roundup of men thought to be leading narcotics dealers.

Walker, who had been living here for more than a year, was a Vietnam veteran who had been working on a novel that included information about drug smuggling. Radelat, who was visiting Walker, reportedly had boasted of his own drug use.

The restaurant manager and a waiter have told Mexican police that Caro Quintero's men first beat and kicked the two Americans in the restaurant's main dining room.

Then the Americans were dragged into a storage room, where Caro Quintero and eight of his men stabbed them with knives and ice picks for more than half an hour, the witnesses said. The restaurant's night watchman told police that he later found a large pool of blood in the storage room.

The manager and the waiter differed on how the Americans looked when they were taken from the restaurant. The manager thought that the Americans were unconscious and possibly dead when Caro Quintero and his group pulled jackets over the victims' heads, placed them in the back of a car and drove away. The waiter said instead that the Americans were led away walking, and that he heard one of them say in English, "I can't see."

In any case, relatives by now have given up hope of finding the two alive.

"At this point, we're just looking for the bodies," said Walker's wife, Eve, who returned to Mexico from Roseville, Minn., where she had been living with the couple's two daughters, because she feared the police were not seriously investigating the disappearance.

The witnesses at La Langosta said that Caro Quintero's gunmen accused Walker and Radelat of working for the DEA after searching them and finding passports or other documents identifying them as U.S. citizens. The four Jehovah's Witnesses -- one couple from Redding, Calif., and another from Ely, Nev. -- were abducted Dec. 2, possibly because their house-to-house visits in search of converts were viewed as a possible cover for drug investigations, Mexican officials and some U.S. diplomats said.

The drug traffickers "have acted with extreme violence against people from the United States because of investigations by the DEA," Gov. Alvarez said in an interview.

Nevertheless, relatives of Walker and Radelat, and some U.S. officials, were cautious about accepting this explanation of the disappearances. They suggested that Mexican officials might have found it convenient to pin all the disappearances on narcotics traffickers who now are in jail, thus allowing the government to claim that the problem has been solved and that it is again safe for tourists to come to Guadalajara.

"They want to be able to say that all of the disappearances have been cleared up," Dr. Felipe Radelat, father of Alberto Radelat, said. He said he was looking for "corroboration" of the accounts given by the witnesses.

This skepticism has been fueled by the delay of more than a month in obtaining statements from the restaurant employes. It was known early in the investigation that Walker and Radelat had been in the area around La Langosta, a small establishment with a high, thatched roof, because Walker's car was found nearby. A taxi driver also said early that he saw the two Americans walking into the restaurant.

In addition, the initial investigation by the Jalisco state police was incomplete at best, and part of a cover-up at worst, according to the victims' relatives and U.S. officials familiar with the case. Agents of the state police, and of virtually every other Mexican law enforcement agency, have been implicated as paid guardians of this country's multibillion-dollar drug industry.

At one point a Jalisco state police officer virtually admitted to Eve Walker that he could do little to help her. He urged her to ask the U.S. Consulate to seek help from the Mexican federal police and from DEA, according to Walker and a Cable News Network correspondent whose crew filmed her conversation with the police officer on March 12.

From the beginning, Mexican officials insinuated to relatives of Walker and Radelat that the two victims had been working for DEA. The DEA denied that either man was working for it, but several circumstances may have led Mexican narcotics traffickers to believe that the two were informers.

Walker, a Vietnam veteran with a 50 percent disability pension, had been living in Guadalajara since November 1983 while working on a novel. The book included a subplot about smuggling cocaine from Mexico to the United States, and a folder was found among his belongings containing clippings and notes about the Mexican drug trade.

Several sources suggested that Walker, a former journalist, may have gone to La Langosta to gather information about what a reputed drug kingpin's restaurant looked like.

Radelat, who had known Walker for more than a year, was visiting Guadalajara to check out the dental school. He had attended dental school in Monterrey, Mexico, from 1980 to 1982 and was considering resuming his studies in Guadalajara. He was staying at Walker's apartment, and the two of them had made a trip to the resort of Puerto Escondido in early January.

Radelat had been working for his father, a family practitioner, as a laboratory technician since dropping out of the Monterrey dental school. He had left that school because he had run out of money to pay his tuition and would not allow his family to pay it, according to his father.

An acquaintance who asked to remain anonymous said that Radelat, who was single, had partied heavily and boasted to friends about his drug use. Radelat's father acknowledged that his son once had a drinking problem, but denied strongly that he used drugs.

If Walker had been asking questions about the drug trade around Guadalajara, and if Radelat was using narcotics, it is easy to see how the two might have been mistaken for DEA agents. They also frequented a hamburger joint called Uncle Sam's Kitchen that was a favorite of U.S. Consulate employes.

"I think that John very innocently stumbled into a set of circumstances that made him appear to be a DEA informer," Eve Walker said.