CHICAGO — The American interrogator was afraid. Of what and why, he couldn’t say. He was riding the L train in Chicago, and his throat was closing.

In Iraq, when Tony Lagouranis interrogated suspects, fear was his friend, his weapon. He saw it seep, dark and shameful, through the crotch of a man’s pants as a dog closed in, barking. He smelled it in prisoners’ sweat, a smoky odor, like a pot of lentils burning. He had touched fear, too, felt it in their fingers, their chilled skin trembling.

But on this evening, Mr. Lagouranis was back in Illinois, taking the train to a bar. His girlfriend thought he was a hero. His best friend hung out with him, watching reruns of “Hawaii Five-O.” And yet he felt afraid.

“I tortured people,” said Mr. Lagouranis, 37, who was a military intelligence specialist in Iraq from January 2004 until January 2005. “You have to twist your mind up so much to justify doing that.”

Being an interrogator, Mr. Lagouranis discovered, can be torture. At first, he was eager to try coercive techniques. In training at Fort Huachuca, Ariz., instructors stressed the Geneva Conventions, he recalled, while classmates privately admired Israeli and British methods. “The British were tough,” Mr. Lagouranis said. “They seemed like real interrogators.”

But interrogators for countries that pride themselves on adhering to the rule of law, such as America, Britain, and Israel, operate in a moral war zone. They are on the front lines in fighting terrorism, crucial for intelligence-gathering. Yet they use methods that conflict with their societies’ values.

The border between coercion and torture is often in dispute, and the American government is debating it now. The Bush administration is nearing completion of a new executive order setting secret rules for CIA interrogation that might ban waterboarding, a practice that simulates drowning. Last September, President Bush endorsed an “alternative set of procedures,” which he described as “tough,” for questioning high-level detainees. And in Iraq last month, General David Petraeus, the top U.S.commander, warned troops that the military does not sanction “torture or other expedient methods to obtain information.”

The world of the interrogator is largely closed. But three interrogators allowed a rare peek into their lives — an American rookie who served with the 202nd Military Intelligence Battalion and two veteran interrogators from Britain and Israel. The veterans, whose wartime experiences stretch back decades, are more practiced at finding moral balance. They use denial, humor, indignation. Even so, these older men grapple with their own fears — and with a clash of values.

That clash, said Darius Rejali, a political scientist who has studied torture and democracy, can torment interrogators: “Nothing is more toxic than guilt, which is typical with democratic interrogators. Nazis, on the other hand, don’t have these problems.”

For Mr. Lagouranis, problems include “a creeping anxiety” on the train, he said. The 45-minute ride to Chicago’s O’Hare airport “kills me.” He feels as if he can’t get out “until they let me out.” Mr. Lagouranis’s voice was boyish, but his face was gray. The evening deepened his 5 o’clock shadow and the puffy smudges under his eyes.

Not long ago in Iraq, he felt “absolute power,” he said, over men kept in cages. Mr. Lagouranis had forced a grandfather to kneel all night in the cold and bombarded others in metal shipping containers with the tape of the self-help parody “Feel This Book: An Essential Guide to Self-Empowerment, Spiritual Supremacy, and Sexual Satisfaction,” by comedians Ben Stiller and Janeane Garofalo. (“They hated it,” Mr. Lagouranis recalled. “Like, ‘Please! Just stop that voice!’ “)

Now Mr. Lagouranis’s power had dissolved into a weakness so fearful it dampened his upper lip. Sometimes, on the train, he has to get up and pace. But he can’t escape. James, an amiable man with a red-to-white beard, shook his head when told of Mr. Lagouranis: “He’s full of self-pity.”

James, 65, was one of Britain’s most experienced interrogators in Northern Ireland. Starting in 1971, James said, he worked for the Special Branch of the Royal Ulster Constabulary, interrogating Irish nationalists Gerry Adams, Bobby Sands, and others whom the British government suspected of being terrorists. Peter Taylor, a leading historian of the conflict in Northern Ireland, said he believes “James’s account is entirely credible.”

Late one night in 1993, three Irish Republican Army gunmen crept up to James’s door. “They came to get me,” James said. Given a 20-minute warning following a tip to the RUC, he and his wife escaped, ultimately to an island in the Mediterranean. James agreed to talk if his last name and location were withheld. “They haven’t a clue I’m here.”

Driving along winding, stony roads, past goats and grapevines, James had this advice for Mr. Lagouranis in Chicago: “You’ve got to get up and get on with it — that’s what we did.”

James had had no training, but the 18-hour days that made his neck ache taught him what he needed: good rapport, good intelligence, great fear. “Yes, a bloke would get a cuff in the ear, or he might brace against the wall. Yes, they had sleep deprivation,” he said. “But we did not torture.”

Once, IRA leader Brendan Hughes claimed James had cocked a gun to his head. James does not deny it. “You fight fire with fire,” he said, the memory igniting his blue eyes.

He noted that the sectarian killings dropped off: “If it’s going to save lives, you’re entitled to use whatever means you can.” How do you fight bad guys and stay good? “You don’t. You can’t.”

The only interrogation James regrets was of Patrick McGee, under arrest for IRA activity. McGee did not crack, which meant he would go free. As McGee walked out, “he just stared at you,” James recalled. “Evil was hanging out of him.” James spat in his face. “He never even blinked. It was not satisfying, it was humiliating. I lost my cool.”

James stopped his car at the edge of the ocean. According to Greek mythology, a god frolicked on this beach. Vacationers drank iced coffee and oiled the air with coconut lotion. But James seemed to be somewhere else, cloudy and turbulent, in his head.

“My friend once saw a guy planting a bomb,” he said. He laughed. “My friend tied a rope around the guy’s ankle, and made him defuse it. Now t hat’s how to deal with a ticking bomb.”

“All of Iraq was a ticking time bomb,” Mr. Lagouranis said, downing his fourth of seven beers. He had joined the Army before September 11, 2001, to learn Arabic. He didn’t expect to go to war.

He was sitting on a night off at the California Clipper bar, where he works as a bouncer. The bartender joked that Mr. Lagouranis should be tougher on customers: “You should ‘go Abu Ghraib.'”

At Iraq’s Abu Ghraib prison, the site of the 2003–04 abuse scandal, Mr. Lagouranis used to relax in the old execution chamber. He and a friend would sit near the trapdoor and read the Arabic scratched into the wall. They found a dirty brown rope. It was the hangman’s noose. “If there is an evil spot in the world, that was one of them,” Mr. Lagouranis said.

At Abu Ghraib and sometimes at the facilities in Mosul, north Babil province and other places where Mr. Lagouranis worked, the Americans were shot at and attacked with mortar fire. “Then I get a prisoner who may have done it,” he said. “What are you going to do? You just want to get back at somebody, so you bring this dog in. Finally, I got you.”

Mr. Lagouranis’s tools included stress positions, a staged execution and hypothermia so extreme the detainees’ lips turned purple. He has written an account of his experiences in a book, “Fear Up Harsh,” which has been read by the Pentagon and will be published this week.

Stephen Lewis, an interrogator who was deployed with Mr. Lagouranis, confirmed the account, and Staff Sergeant Shawn Campbell, who was Mr. Lagouranis’s team leader and direct supervisor, said Mr. Lagouranis’s assertions were “as true as true can get. It’s all verifiable.”