When FBI agents interviewed accused spy Larry Wu-Tai Chin shortly before his arrest Nov. 22, the retired CIA analyst declined to sign a written confession, explaining, as one agent recalled in court last week, "That would be evidence coming from the horse's mouth, and I would be the horse."

Still, Chin obligingly provided agents with an outline of his alleged 30-year career of spying for the Chinese -- evidence that, although it will be related secondhand by FBI agents, still comes, in Chin's words, "from the horse's mouth." A federal judge ruled Friday that the statements could be used at Chin's trial next month.

Chin was not alone in agreeing to talk to the FBI. One of the most remarkable aspects of the recent rash of spy cases has been the FBI's apparent success in convincing those suspected of espionage to confess their alleged involvement when confronted by agents.

The vast majority of the 11 people arrested on espionage charges last year are alleged to have made incriminating statements to federal investigators. In many of those cases, the admissions constituted the heart of the prosecution's case against the individual.

The interrogation of a suspect -- or interview, as the FBI prefers to term it -- is particularly important in espionage cases because of the murky character of the crime.

By its very nature, espionage is designed to have few witnesses and involves foreign agents who are generally beyond the reach of U.S. law enforcement authorities. In addition, many of the cases involve alleged acts of espionage stretching back as much as 30 years, making it difficult for investigators to gather enough details for a conviction without evidence from the suspects themselves.

All of this makes the agents' chance to question the suspect an especially critical step in any espionage investigation. FBI agents assigned to counterintelligence work receive special training in interviewing spy suspects. Far from crowing about their recent coups in such interviews, however, law enforcement officials are squeamish about discussing their techniques for fear of alerting those yet to be caught to their methods.

But court documents and testimony in an array of the spy cases of the last year have offered a rare glimpse into how the FBI goes about that delicate task, in which the difficulties inherent in obtaining a confession are further complicated by the need to make certain -- at the risk of losing the chance to use the evidence in court -- that the suspect's constitutional rights have not been violated.

In the Chin case, for example, the three agents who questioned the former Central Intelligence Agency analyst repeatedly told him he was free to ask them to leave or to call a lawyer, according to testimony last week. When Chin asked at one point what a lawyer would tell him, one of the agents candidly advised Chin that a lawyer would warn him not to talk.

This information about Chin's rights, however, was sandwiched between statements that, should he choose to assert those rights, he would lose his only chance to cooperate. Agent Mark R. Johnson testified in Alexandria federal court Friday that he told Chin, "You don't have to talk to us if you don't want to. Anything you say is voluntary." At that point, Johnson added, "Another agent entered the conversation and said, 'Mr. Chin, this is your only opportunity to talk to us.' "

In addition to the agents' presentation of the interview as a one-time-only offer, they suggested to Chin -- while being careful not to make any promises -- that he might receive more lenient treatment and less publicity if he were to cooperate.

When Chin asked the agents why he should talk, Johnson testified, they presented two examples, one of an individual who had cooperated, and one who had not, and asked, "Now if you were the Department of Justice, which one would you have more sympathy for, the individual who slammed the door in the agent's face or the one who said what had happened and said he was sorry?"

In fact, as defense lawyer Jacob A. Stein pointed out in cross-examining a second FBI agent, "Nobody said to the Department of Justice, 'He has availed himself of his last opportunity and we are wondering . . . whether you would be lenient with him.' "

Indeed, Stein noted, even as the agents were telling Chin that this was his only chance to explain his actions before they took the case to the Justice Department, department lawyers were standing by to learn the results of the interview and authorized Chin's arrest later that night.

FBI agents have used these and other techniques in other espionage cases. Jerry Alfred Whitworth, who is to stand trial in San Francisco next month on charges of spying for the Soviets with John Anthony Walker Jr., complained that agents subjected him to "psychological warfare" when they questioned him and searched his home last May, hours after Walker's arrest.

Whitworth said agents told him, "You can tell us what you know, get it off your chest," and they suggested that if he did not, they would obtain a search warrant, take Whitworth to jail, and keep his wife, Brenda Reis, away from their home.

"I felt desperate," Whitworth testified last month. When agents asked him to sign forms permitting them to search his home and waiving his rights to remain silent and to consult a lawyer, "I just sort of felt that this would give me some relief, if I signed this thing."

U.S. District Court Judge John P. Vukasin later rejected Whitworth's bid to keep prosecutors from using his statements at his trial.

John Walker's brother, convicted spy Arthur James Walker, whose admissions to the FBI constituted the centerpiece of the government's case against him, testified that an FBI agent had told him that "if the FBI could tell the Justice Department that I was cooperative, that it would help me."

FBI agents denied making any such promises, U.S. District Court Judge J. Calvitt Clarke Jr. upheld the use of the confession, and Arthur Walker is now serving a sentence of life in prison.

The government's batting average in being allowed to use the statements made by alleged spies has been extremely high. One relatively minor loss came in the case against John Walker, who pleaded guilty to espionage Oct. 28. After his arrest May 20, John Walker told agents he did not want to answer any questions without an attorney present, meaning agents had to stop interrogating him.

While Walker was being held at the FBI's Baltimore office, however, agents showed him a copy of a "Dear Friend" letter Walker had left in a bag of classified documents for his Soviet contacts -- letting Walker know for the first time that the FBI had located the bag.

U.S. District Court Judge Alexander Harvey II ruled that showing Walker the letter was the "functional equivalent of interrogation" and refused to let prosecutors use a statement that Walker made to agents an hour later.