For two weeks in April, Marine Cpl. Robert J. Williams of the Bronx seemed destined to become the star witness in the espionage trial of a fellow guard at the U.S. Embassy in Moscow.

On April 2 Williams had given investigators a detailed account of how Cpl. Arnold Bracy broke down in tears as he was leaving the Soviet capital, supposedly describing how an embassy cook had seduced him into committing acts of espionage for the KGB. That, plus Bracy's own statement of complicity, seemed to make for an ironclad case of espionage.

But then both men suddenly recanted their statements, leaving what was one of the military's most celebrated spy cases a shambles.

Yesterday the case took another strange turn. The Marine Corps said it has charged Williams with 11 counts of "false swearing" -- accusing him of telling lies in much of his original testimony to investigators. These allegations were filed despite statements by top Marine Corps officials that they continue to believe the now-dismissed charges that guards welcomed Soviet agents into the embassy.

The only known basis for the belief that Soviet agents were permitted into the embassy is the testimony under interrogation provided by Bracy, who initially described himself as a "lookout" for Sgt. Clayton J. Lonetree, who is charged with espionage in the case. A senior U.S. intelligence official said there is no independent evidence confirming that any Soviet agents were allowed into secret areas of the U.S. Embassy in Moscow by the Marines.

Defense lawyers representing the accused Marines have expressed outrage that the government would charge Williams with falsely swearing statements elicited in extended interrogations by agents of the Naval Investigative Service (NIS). Defense lawyers argue that NIS agents tricked and cajoled the former guards into making false statements about the two principal suspects in the investigation.

A spokesman for the Marine Corps said yesterday that some of the new counts of false swearing against Williams "appear contradictory." But investigators decided that Williams had told so many versions of what happened that a pretrial hearing officer should try to resolve the contradictions. Capt. T.W. Bjerke, Williams' attorney, said he has not been formally served with copies of the charges, adding that his client will probably plead not guilty.

Like Lonetree and Bracy, Williams asserts that he was tricked into making incriminating statements during long interrogations by NIS agents. The Marines contend that the use of polygraphs, protracted and threatening interrogations, and the use of various ruses by the agents led them to make false statements.

Senior military officials have rejected these criticisms of the investigation.

Retiring Marine Corps Commandant Gen. P.X. Kelley said Sunday on CBS News' "Face the Nation" that the corps is looking into criticisms of the NIS. "But thus far, we haven't seen anything that would give any credence at all to any suggestion that the Naval Investigative Service was just not totally professional in the way they handled this case," he said.

The investigation began as what Lonetree later would describe as a "private conversation" Dec. 14 at a Christmas party at the U.S. Embassy in Vienna. The thin young Marine wandered up to an embassy official who was a Central Intelligence Agency employe.

What Lonetree laid out was a remarkable tale: During his previous 18-month tour in Moscow he had been secretly working with several Russians in a scheme that he said could pluck Edward Lee Howard, the notorious CIA turncoat, from the Soviet Union and whisk him back into the United States.

At the time Lonetree was under increasing pressure from a Soviet agent in Vienna named George to return to Moscow. He had recently joined an alcohol treatment program to deal with a drinking problem. By his own subsequent account, he was afraid he was "getting in deeper and deeper" in his dealings with the Soviets.

That brief meeting between the CIA agent and 25-year-old Marine from Minnesota set off a massive investigation that led to questioning of hundreds of former embassy guards and to public pronouncements from the president and his top advisers about the Soviets' "outrageous" conduct.

Six months later, the most serious of the charges that military prosecutors filed -- that Lonetree and Bracy allowed Soviet agents into the embassy -- have been dismissed. What remain are allegations about two dozen Marines suspected of breaking rules against fraternization with Soviet women, the new charges against Williams and the initial espionage charges against Lonetree. The latter could send the son of an American Indian activist to prison for life, but these charges are based almost entirely on what Lonetree told the CIA agent at the holiday party in Vienna.

Defense lawyers have ridiculed the entire investigation as "a world-wide wild-goose chase" that was bungled by NIS agents, members of the Navy Department's largely civilian police force. Bracy, the Queens, N.Y., Marine accused of being Lonetree's lookout, charged after his release June 12 that the idea that KGB agents got into the embassy was "a scenario . . . cooked up . . . concocted" by NIS agents anxious to prove that he was involved in espionage.

"There never was anyone in that building . . . " Bracy has maintained. "What I was charged with never happened."

"I don't have a reasonable doubt that it happened," countered a top Navy official familiar with the investigation.

Even the sex elements in the case have faltered. Lonetree has not disputed having an affair with Violette Seina, an attractive Soviet who worked as a translator at the embassy. Bracy, a born-again Christian, however, has strongly denied that he had been caught in bed with a former cook at the Moscow Marine House.

Members of key congressional committees are perplexed and have begun to ask the Defense Department to explain how the case that seemed so serious has quickly become so flawed.

"I don't know what to believe," said John Barron, the author of books on the KGB and the Soviet intelligence apparatus, expressing a position that is widespread in Washington.

Several senior State Department, Navy and Marine Corps officials, who agreed separately to discuss the case on the condition they not be named, said that despite dismissal of the entry charges against Lonetree and Bracy, they are convinced that Soviet agents did gain access to the embassy.

The trouble with the legal case, said several of the officials, is that in a military court a suspect's statement alone is not sufficient to convict. Since Bracy, whose statement provided the basis for the entry charge, recanted shortly after seeing a lawyer, the prosecutors said they had nothing to support the charge in court.

Investigators had hoped to find others in the 28-member Moscow Marine detachment who might support the charge. Until Williams recanted on April 19, he was expected to give that support.

Not only did other Marines fail to come forward to support the entry account, but what happened to the Marines being questioned may have compounded the prosecution's problems.

Navy and Marine officials deny any wrongdoing by NIS agents. "It's okay for criminal investigators to trick people" into making statements, said one official. "But that's not going to overcome their {the suspects'} free will."

"These Marines may be young, but they know what espionage is," said a top Navy official.

Defense lawyer Michael V. Stuhff, who represents Lonetree, disagreed. "You have to consider that these Marines are young, naive folks who have been trained to respect authority," he said. "Well, to them, these {NIS} agents are essentially wearing the same uniform," and under prolonged questioning they simply forced the Marines to accept their views of the case, he said.

Stuhff and others cite many problems with the NIS investigation, such as that investigators used repeated polygraph tests to try to coerce incriminating statements from the suspects. The investigators -- not neutral polygraph operators -- administered the tests, the lawyers said.

To Stuhff, "the real, real weakness of the NIS" is what he describes as its tendency to rely on interrogations and confessions to support its cases.

That may work well in many NIS cases, which typically involve thefts aboard ships and petty crimes on military bases, but it is insufficient in cases as serious as espionage, he said.

Navy officials respond that NIS agents followed approved procedures in the Moscow inquiry. The interrogations, often conducted in motel rooms near military bases, were not overly long and always followed approved formats, one senior official said.

Subjects repeatedly were informed of their rights and given breaks from questioning, which was not as intense as the defense lawyers maintain, the official said.

The military officials do not dispute extensive use of polygraphs in the inquiry, acknowledging that some Marines were interviewed as many as 10 times while attached to the machines. But repeated use of the machines was required, the officials said, because the tests were showing the Marines to be evasive on key points.

"I don't think anything we did was an overreaction," said one senior Marine. "We would have been faulted if we had not moved quickly and put these guys in jail."