WASHINGTON — Thousands of job applicants come to FBI offices all across the country every year, eager to work for the top law enforcement agency in the U.S.

But many of them have their hopes dashed, and it’s not because of their work experience or education or criminal records. They’re turned down because they’ve failed their polygraph tests.

The FBI’s policy of barring job candidates who fail their polygraph tests clashes with the view of many scientists that government agencies shouldn’t be relying on polygraph testing to decide whether to hire or fire someone. Experts say polygraph testing isn’t a reliable indicator of whether someone is lying – especially in employment screening.

Further, a little-known technical glitch in one of the leading polygraphs that the bureau and many other government agencies have used could give applicants who fail polygraphs even more reason to assert that they were inaccurately and unfairly labeled liars.

“I was called a lazy, lying, drug dealing junkie by a man who doesn’t know me , my stellar background or my societal contributions,” wrote one black applicant in Baltimore, who said he was told he qualified for a job except for his polygraph test failure. “Just because I am young and black does not automatically denote that I have ever used any illegal drugs.”

Government agencies use polygraph testing not only to weed out job applicants but also to question criminal suspects and to determine whether sex offenders are complying with psychological treatment or probation.

Although all polygraph testing is controversial, many scientists are highly critical of its use in job screening, saying it’s especially prone to inaccuracies because the questions are often more vague than they are in criminal investigations and therefore they’re more likely to provoke reactions from the innocent that might seem like deception.

Adding to the skepticism, polygraphers have documented problems with the measurement of sweat by the LX4000, a polygraph that the FBI and many other federal agencies and police departments across the country have used, McClatchy found. Polygraphers also interpret measurements of respiration and blood pressure for their decisions on whether someone is lying, but many see the sweat measurement as especially indicative of deception. The manufacturer of the LX4000, Lafayette Instrument Co. Inc., describes the problem as rare but it isn’t able to specify what that means. The company also points out that other polygraphs that use the same technology might have the problem as well.

“The potential for occasional differences to be observed was already clearly described in the published literature, not limited to the Lafayette polygraph system, and anecdotal experience confirms it is an occasional occurrence,” the company said in a statement to McClatchy.

But Gene Iredale, a San Diego lawyer, said the problem could mean that agencies such as the FBI are “making decisions in criminal cases and are taking away security clearances on the basis of completely inaccurate information. The polygraph is speculative to begin with; that’s why it’s not admissible in court.”

A spokeswoman said the bureau, which has polygraphed applicants since 1994, “continues to find it a relevant and necessary tool in both employment and operational related matters.”

The spokeswoman, Kathleen Wright, said the bureau wouldn’t respond to questions about the LX4000 nor reveal the number of people who failed each year, although she acknowledged that if an applicant doesn’t pass a polygraph “then the applicant is normally not hired.”

Records obtained by McClatchy show the FBI polygraphs about13,000 people a year for job screening, criminal investigations and national security inquiries. According to records, the bureau has at least a 30 percent failure rate in its job screening. As many as 40 percent of special agent applicants don’t get the job because of their polygraph test results.

The bureau isn’t alone in its reliance on polygraph testing. A contractor fired 10 Drug Enforcement Administration court interpreters in San Diego after the DEA said they’d failed their polygraphs or refused to take them , according to a federal lawsuit Iredale filed against the U.S. government over the decision. The Philadelphia Police Department says on its website that it won’t hire someone who doesn’t pass its polygraph.

Other police departments, such as Kansas City, Mo., Sacramento, Calif., and Raleigh, N.C., say they don’t rely entirely on a polygraph test to determine whether they should hire someone.

“If someone shows deception and we’re not able to clarify it, then the investigator puts that in their report and that’s something we look into,” said Kathy Lester, who oversees the Sacramento Police Department’s hiring. “It’s not just based on the polygraph, but we’ll have other corroborating information.”

However, even those who say they don’t have a strict policy of barring failing applicants were hard-pressed to say what they’d do if they were faced with a failure without a confession of wrongdoing.

“We have to feel comfortable hiring someone,” said Lester, who estimated that her department would polygraph about 200 people this year for positions ranging from police officers to firefighters to clerks. “If we feel somebody is deceptive based on the totality of the circumstances, then we can’t hire them.”

Despite its strict policy, the FBI gives some applicants the opportunity to take another test, but it doesn’t disclose the number.

James Wedick, a retired FBI agent of 34 years who’s now a private investigator in Sacramento, called the bureau one of the “worst culprits” in its reliance on polygraph testing. He said he’d repeatedly seen highly qualified people turned away who’d be excellent law enforcement officers. The stigma follows them even if they try to apply elsewhere, because police departments and federal agencies often weigh whether someone previously failed a polygraph, even if they haven’t tested the person themselves.

“Many of these people work their entire adult lives to get a job with the bureau,” he said. “These are people who haven’t done anything wrong and they’ve got stellar resumes and they’re told, ‘Sorry. You can’t get a job because of the polygraph.’ It’s ludicrous.”

Documents obtained by McClatchy demonstrate that the bureau is rejecting people even as the applicants insist they’ve never done anything wrong. The documents, which were given to McClatchy by polygraph researcher Katelyn Sack, also show that some applicants view the rejection as illegal discrimination. In fact, half of a dozen applicants who filed the complaints McClatchy obtained thought they were discriminated against because they’re African-American.

"I believe I was discriminated against as a black female,” wrote one woman who applied to the Memphis, Tenn., FBI office and was told she’d failed two polygraphs.

The first polygrapher said the woman seemed to be lying about drug use. After she was granted another test, the second polygrapher said she passed. A supervisor overturned that conclusion.

In all the cases reviewed, the bureau countered that the polygraph tests revealed the applicants were hiding some sort of wrongdoing that wasn’t discovered during the usual background investigation.

In Dallas, for instance, one applicant accused a polygrapher of asking whether he was an Israeli spy because he’s Jewish and of Iranian descent. The polygrapher, however, said the polygraph results showed him to be deceptive in response to questions about spying. “Other FBI officials confirmed the accuracy of the polygraph reading,” the records say.

The FBI acknowledged that the polygrapher did mention an Israeli spy agency, “as he typically discusses foreign intelligence services with applicants.”

In response to another complaint filed by a U.S. citizen of Iranian descent, the bureau said “the outcome of the polygraph examination was based solely on (complainant’s) reaction to the questions posed him.”

The polygrapher who first tested the man concluded that he passed. Her supervisor, however, said the bureau couldn’t determine whether he was lying, calling him “inconclusive.”

When he was tested again, said the applicant, who applied in Albany, N.Y., he was accused of trying to beat the polygraph test and told he was lying about links to terrorists.

The FBI denied discriminating against the applicants, who filed the complaints from 2006 to 2010. The bureau said supervisors who reviewed the polygraphers’ determinations didn’t know the characteristics of job applicants and therefore couldn’t be biased.

Government agencies often successfully assert that national security and law enforcement interests trump an applicant’s right to sue over a polygraph test. They persuade courts of polygraph testing’s usefulness by pointing to confessions of wrongdoing they elicit during the tests.

However, Sacramento attorney David P. Mastagni called the technical glitch of the LX4000 a “real, significant legal issue” that could give people who have been denied jobs because of polygraph tests a legitimate reason to sue. Mastagni, who represents law enforcement officers in employment disputes and criminal cases, said manufacturers could be accused of not disclosing a product defect if they were aware of such a flaw and didn’t do enough to fix it.

“If someone inaccurately got labeled a liar, that’s a big deal. This is not a crybaby issue,” he said. “It could be lots of people lost out on a job.”

Tish Wells contributed to this article.