Light filters through the heavy morning clouds and into the cramped waiting room, shining on Lawrence J. Mangan as he shifts in his chair, waiting to be grilled.

Just before 9 a.m., Darryl L. DeBow comes for him. They walk through a storage room and into a windowless office, where DeBow will attach Mangan to a computer, via two chest straps that will monitor his breathing and put a blood pressure cuff on his arm and metal plates on his fingertips to gauge perspiration.

Mangan has gone through an 800-question psychological exam and taken a drug test. He now faces his final barrier to employment at the Leesburg Police Department: a lie detector test.

He'll be one of about 1,000 people tested this year by Northern Virginia Pre-Employment and Polygraph Services, which is affiliated with the Virginia School of Polygraph, based in downtown Leesburg.

If he passes, Mangan can quit his job with the Vienna Police Department, with its alternating schedule of nights and days, and work saner shifts closer to home and his wife and three children.

"We've weeded out a lot of bad candidates. They come in spit shined and look good on paper, then fall apart during the polygraph," DeBow said in an interview. "Sex crimes, theft from employers, falsification of records. . . . You name it, people have done it."

The beginning of the test is informal. No wires, no digital monitors. Just a couple of guys talking. DeBow inquires about Mangan's 21 years as a New York City correctional officer and asks him to rate how happy his childhood was and how many drinks he has in a given week.

"We don't work with angels here," DeBow told him. "You got to give me the 100 percent truth. You got to get it out there."

Before he entered the controversial field of polygraphy, DeBow was a Loudoun County sheriff's deputy. In the early 1990s, shortly after a promotion to sergeant, he fell off a ladder during a SWAT team drill and landed on his back. His injuries caused chronic pain and confined him to a desk job. DeBow went to polygraph school and returned to the department as an examiner.

"When one door shuts, another door opens," he said. "I was given a second chance."

In 2003, he bought the Virginia School of Polygraph, one of 19 schools in the world accredited by the American Polygraph Association, and moved the headquarters from Virginia Beach to Leesburg. The school annually trains 20 to 35 examiners, who come from as far as Costa Rica and Canada.

DeBow and four other examiners administer tests, monitoring the activities of convicted sex offenders, aiding criminal investigations, testing potential hires for local police or fire departments and checking the fidelity of clients' potential spouses.

"Everybody has that deep, dark little secret that they want to keep hidden," DeBow said. His job is to expose buried misdeeds through his probing questions and, later, through his technological fluency.

The informal, introductory questions continued:

Did you ever commit the act of burglary?

Assault and battery? Domestic abuse?

Rape, forcing someone to have sex who was drunk or drugged?

Exposing yourself or peeping in someone's window?

Petty larceny; theft of anything?

After a string of no's, Mangan hesitated at the last question. The hum from the computer filled the room.

"I guess when I was a kid, maybe candy," Mangan finally said.

"When you lie, you have what is called a sympathetic response; your body goes into fight-or-flight mode," DeBow said. "It affects the pulse rate, blood pressure, respiratory and galvanic skin response [sweatiness]. We measure these things."

"That's nonsense," said Drew C. Richardson, a former FBI agent with a PhD in physiology. He has testified before the Senate, challenging the government's use of polygraph testing. "There isn't an isolated 'lie response,' " he said.

Such emotions as anger, surprise or revulsion also can trigger similar physiological responses, Richardson said. When a job is on the line, someone could be responding in fear "to the consequences of being branded a liar, rather than being caught in a lie," he said.

Have you supported any country other than the U.S.?

Are you prejudiced?

Have you ever been the focus of a criminal investigation?

What do you think the worst thing you've ever done in your life is?

Polygraph testing has long been called "quackery" or "pseudo-science" by many scientists as well as by burned test-takers. In 1988, Congress outlawed the use of polygraph exams by employers in the private sector amid concerns about unwarranted invasions of privacy and the potential for false but potentially ruinous indications of deception.

Nevertheless, polygraph testing has remained legal in the public sector and is on the rise among federal agencies and local police departments, which rely on them to screen applicants and monitor the activities of their employees, said W.E. Chittenden, president of the Virginia Polygraph Association.

"We don't use it as a sole source of ruling someone out. It's just another tool to help qualify and rank candidates," said Leesburg police Lt. Jeff Dube, who is responsible for the department's hiring and recruitment.

Lest Mangan have any doubts about the test's accuracy, DeBow set up a drill. After hooking Mangan up with the chest straps -- pneumographs -- the blood pressure cuff and the fingertip clips, DeBow asked the applicant to pick a numbered card and lie when DeBow guessed the correct number. Judging from the spikes on the on-screen chart, DeBow correctly identified the number Mangan picked and then denied plucking.

Mangan looked impressed. Then he looked nervous. The real test was about to begin. DeBow ran through a list of questions he was going to ask, then went through them again, this time for real:

Is your first name Lawrence? Yes.

Have you ever detected a serious undetected crime? No.

Have you ever committed a sexual crime you could have been arrested for? No.

Have you ever stolen anything worth over $100 in value? No.

Partway through, DeBow asked Mangan to breathe more evenly.

Have you told the complete truth to the best of your knowledge?

Yes.

"Okay, you can relax."

Back in the waiting room, Mangan was anything but relaxed.

"It's the weirdest feeling, because you feel your heart in your throat and your heart's pounding," he said. Knowing he had a secure job to fall back on took some of the edge off, he said, but he was nervous anyway.

Reviewing the peaks and valleys fed by each of the instruments on the screen, DeBow said he didn't see any problems. His assessment: DeBow passed with flying colors.

"You get very few really clean whistles. It's nice to interview someone like that. It restores your faith in humanity," he said.

Maybe there are angels in this business after all.

Tomorrow, Mangan will start his new job as an officer with Leesburg police.

A computer screen displays results as Lawrence J. Mangan undergoes a polygraph examination at a testing center in downtown Leesburg.