It seemed like a routine polygraph screening. Mark Mallah and his colleagues, members of an FBI counterintelligence unit in New York, were hooked up to lie detector machines and quizzed about drug use, contacts with foreigners and other subjects deemed vital to their roles in protecting national security.

The test turned out to be anything but ordinary for Mallah. The 10-year FBI agent said he was accused of being deceptive on the lie detector examination, prompting a suspension from his job and a full-scale investigation that included 24-hour surveillance and interrogations of family and friends.

When he was finally cleared and reinstated 19 months later, Mallah said, he quit.

"I didn't have any desire to work for an organization that would do that to me," said Mallah, who left the FBI in 1996 and now practices law in San Francisco. "They never produced any evidence or came forward with anything, but the polygraph still undermined my career. . . . I was effectively ruined."

In the wake of charges that veteran agent Robert P. Hanssen had spied for Moscow since 1985, the FBI is embroiled in a debate over how far to expand its use of polygraph tests of employees with access to sensitive information.

Some analysts and lawmakers argue that more aggressive use of the devices might have stopped Hanssen -- who was never required to take a lie detector test during his 25 years with the bureau -- much earlier, possibly limiting the damage he allegedly caused. But skeptics say that allegations such as Mallah's underscore the danger in relying too heavily on polygraph devices, which are not considered reliable enough to be used in court.

Attorney General John D. Ashcroft said the FBI will expand the use of polygraphs on some FBI employees because of "the very important consequences of breaches" in national security. But even as he made the announcement earlier this month, Ashcroft conceded that "the polygraph is not a sure way," and estimated the machines have an error rate of about 15 percent.

Critics argue that such "false positives" can derail careers, and even proponents concede that 2 to 5 percent of the tests are probably inaccurate.

At the CIA, for example, hundreds of employees have been placed on administrative leave or removed from sensitive duties since that agency stepped up its use of lie detectors in the wake of the Aldrich H. Ames spy case in 1994. While most are cleared of wrongdoing and eventually return to their jobs, many remain in a kind of polygraph limbo -- able to go to work but not permitted to do anything, officials say.

At the FBI, meanwhile, about a fifth of job applicants fail preemployment lie detector tests, according to a report to Congress. Few get a second chance and most are denied jobs, officials said.

Polygraph machines, which measure a person's respiration, blood pressure and other physical changes, can be defeated. Ames passed two polygraph exams while spying for Moscow, although investigators later faulted one of those tests as "deficient." Some Web sites brag that people can learn to beat the tests in a matter of hours.

"It's a blackjack with bells and whistles," said Roy W. Krieger, Mallah's former attorney. "It's far more trouble than it's worth. . . . Ames was walking out the door with grocery bags full of stuff and he was passing his polygraphs with flying colors."

Polygraph defenders disagree, arguing that if nothing else, the machines serve as a deterrent and can provide crucial clues in intelligence cases. At the CIA, for example, agent Harold J. Nicholson indicated deception when he said in several polygraph exams in 1995 that he had no unauthorized contacts with foreign spies. Ten months later, Nicholson, a former CIA station chief in Romania, was arrested on charges of spying for Russia.

Sen. Bob Graham (Fla.), the ranking Democrat on the Senate intelligence committee, said lie detectors can be compared to metal detectors, which are more likely to prevent someone from taking a weapon to an airport or public building than to actually catch someone with one.

At the FBI, polygraphs have been mandatory for new employees since 1994, and have been administered to some agents like Mallah because they have access to sensitive information about a secret program or criminal case. Until the Hanssen case, however, Director Louis J. Freeh resisted calls to increase the use of lie detectors, joining many in the FBI's rank and file who fear that inaccuracies and inconclusive results would unfairly damage careers.

The FBI's relatively limited use of polygraphs contrasts sharply with the practice at intelligence agencies such as the CIA and the National Security Agency, which have adopted widespread polygraph policies, and the Department of Energy, where congressional and agency mandates have resulted in tests for nearly 20,000 employees in the wake of concerns over alleged Chinese espionage.

Freeh said in an appearance last week that polygraphs "can work as a deterrent," but also said he feared broad use of the tests could have an impact on morale. "If you remove every person who registers positive, thereby ruining their career in many cases, that's an awfully significant decision to make," he said.

William H. Webster, the former FBI and CIA director who is leading an investigation of the Hanssen case, has said that his panel is likely to recommend increased use of polygraph tests at the bureau.

Polygraphs are used widely by police and prosecutors as interrogation tools, and by defense attorneys as bargaining chips, even as test results remain inadmissible in court because they are considered unreliable. The Employee Polygraph Protection Act of 1988 prohibits most private employers from using the tests to screen applicants.

However, the law does not apply to the government. FBI officials have told Congress that during the first three years of polygraph screening for new employees, 20 percent, or 3,270 of 16,200 prospective hires, were "determined to be withholding pertinent information." But only 1,170 admitted doing so in follow-up investigations.

Mark S. Zaid, a Washington attorney who has filed two lawsuits relating to polygraphs against the FBI and other federal agencies, said that shows that many of those who flunk lie detector exams -- either before being hired or during their careers -- are innocent of deception or wrongdoing. Nervousness, uncertainty over an answer and interrogation tricks by polygraph examiners can trip up honest people, Zaid and other critics say.

"The FBI will go through the same thing they've gone through at the CIA: You're going to see hundreds and hundreds of people falling into administrative limbo," Zaid said. "You've got people in the CIA who have been in that position for years. Their careers are dead, and they've done nothing to deserve it."

Matt Pullman had worked toward becoming an FBI agent for years, finally getting a conditional offer of employment in 1998 after graduating from law school. But Pullman said he ran into problems on the polygraph test because he couldn't recall precisely how many times he had smoked marijuana as a college freshman.

Despite his impressive re{acute}sume{acute}, which includes a position in the Army Judge Advocate General reserve and stints at the United Nations and the U.S. attorney's office in Chicago, Pullman said he was not allowed to retake the test. In addition, he said, officials at other federal law enforcement agencies have indicated that the test result would be held against him.

The FBI disqualifies anyone who has used marijuana more than 15 times in a lifetime, and requires applicants to remember specifically how many times they had done so, according to numerous applicants. Defense attorneys who specialize in polygraph cases say these types of drug questions frequently cause difficulties, because they require a precise answer and then punish people for not being sure of the number.

"I was heartbroken," said Pullman, who practices law in Chicago. "One of the reasons I even went to law school was with the hope of getting an FBI position. It was my dream job and, let's face it, I was a pretty good candidate."

George W. Maschke, who says he was wrongly flunked by an FBI polygraph examiner, runs a Web site called that includes information he claims will help people beat a lie detector test by controlling their breathing, biting their tongues or thinking "exciting thoughts."

Maschke, who calls polygraphy "junk science," said the Ames case shows how the tests can make officials overconfident in their ability to combat espionage.

"The only spies who will get caught are those who are stupid enough to admit it, and it will lull us into a false sense of security," said Maschke, an Army Reserve captain with top-secret clearance. "For the polygraph to deter espionage, the spy has to believe it works, but there is no evidence at all that it does."

But Joseph J. Aronica, a former assistant U.S. attorney who prosecuted espionage cases in Northern Virginia, said polygraphs are particularly important as an "investigative tool" in spy inquiries. In its broader use, Aronica likens the technique to urinalysis drug tests required for many government and private-sector employees.

"The bottom line is that you prosecute espionage cases by polygraphing people," said Aronica, who is in private practice. "In an area like foreign counterintelligence or for anyone with access to highly classified materials, it should present no big issue to subject those people to random polygraphs."

Nick Catrantzos, a former CIA case officer who is operations director at Control Risks Group in McLean, agrees. If a polygraph system is designed well, he said, the benefits far outweigh drawbacks.

"It's a significant deterrent," Catrantzos said. "You don't hear about the successes; you only near about the misfires. . . . If people have comfort in the process and they know there's nothing nefarious in the process, I think the overwhelming majority of complaints will wash away."

Staff writers Vernon Loeb and David A. Vise contributed to this report.