The smoking gun? ''Moans and sighs'' heard as the investigator, a Wal-Mart employee, pressed his ear against a hotel room door inside the local Holiday Inn, according to legal documents. Soon after, the company fired the manager for what it said was improper fraternization with a subordinate.

Wal-Mart, renowned to outsiders for its elbows-out business tactics, is known internally for its bare-knuckled, no-expense-spared investigations of employees who break its ironclad ethics rules.

Over the past five years, Wal-Mart has assembled a team of former officials from the CIA, FBI and Justice Department whose elaborate, at times globe-trotting, investigations have led to the ouster of a high-profile board member who used company funds to buy hunting equipment, two senior advertising executives who took expensive gifts from a potential supplier and a computer technician who taped a reporter's telephone calls.

The investigators - whose résumés evoke Langley, Va., more than Bentonville, Ark., - serve as a rapid-response team that aggressively polices the nation's largest private employer, enforcing Wal-Mart's modest, by-the-books culture among its army of 1.8 million employees (more than 15,500 in Utah).

Wal-Mart is certainly not the only company, or even the first, to investigate its employees, a practice used widely in corporate America to guard against fraud and protect trade secrets. That said, despite the retailer's folksy Arkansas image, few companies are as prickly - or unforgiving - about its employees' wayward behavior, a legacy of its frugal founder, Sam Walton, who equated misconduct with inefficiency that would cost customers money.

No case better demonstrates the company's prowess - or, ex-employees say, its ruthlessness - than the exhaustive investigation of Julie Roehm and Sean Womack, two former top Wal-Mart marketing executives.

After Roehm sued Wal-Mart for wrongful termination, the company disclosed the results of the investigation a couple of weeks ago in a detailed and at times salacious counter-lawsuit. Investigators obtained records that they said showed the two executives - both married - had engaged in a sexual affair, accepted free meals from an advertising agency vying to win Wal-Mart's business and began negotiating a deal to leave Wal-Mart to work for that agency.

Last week, Roehm called Wal-Mart's investigation ''a smear campaign'' intended to destroy her reputation and, in a nod to Wal-Mart's investigative firepower, said the company had outgunned her with ''ex-CIA operatives'' and ''former FBI men.''

The Wal-Mart investigation was striking in its scope. Lawyers for Wal-Mart subpoenaed Womack's wife, Shelley, compelling her to give sworn testimony about how she discovered a sexual relationship between her husband and Roehm. They prompted her to turn over dozens of embarrassing e-mail messages that her husband had sent to Roehm from a private account.

''I miss you ridiculously,'' began one of the e-mail messages from Roehm to Womack. ''I hate not being able to call you or write you. I think about us together all the time. Little moments like watching your face when you kiss me.''

Wal-Mart investigators also persuaded the top executives at a major advertising agency, Draft FCB, and its parent company, the Interpublic Group of Cos., to turn over hundreds of confidential e-mail messages, dinner receipts and notes from meetings. One revelation was that Roehm accepted a case of Effen vodka from the chief executive of Draft FCB, valued at nearly $400, calling the gift, which violated Wal-Mart's policies, ''a HUGE hit,'' in a thank-you e-mail message.

Roehm and Womack have denied they engaged in a sexual relationship or did anything wrong. Womack did not respond to phone messages.

Kenneth Senser, a former top official at the CIA and FBI who runs Wal-Mart's security department, said cases such as these show that Wal-Mart is determined to consistently enforce its employment policies, no matter how high the rank of the workers involved. Womack and Roehm, for example, were senior executives with six-figure salaries.

''It's been very clear from these investigations that the company has taken a definitive stand,'' said Senser, who interviewed Roehm and Womack before they were fired in late 2006. ''If it's a senior vice president, or cashier in the store, we are going to look at the allegations the same way - and not give somebody a pass.''

Senser, 47, and his staff of roughly 400 investigate allegations of misconduct, guard Wal-Mart executives and prepare for potential crises of all kinds, such as hurricanes or terrorist attacks. (During Hurricane Katrina, the team established an emergency response center inside Wal-Mart's headquarters, filled with flat-screen televisions that resembled a center used by the FBI.) Their backgrounds are impressive, if not slightly intimidating. Senser was a senior officer in the CIA's office of security, which was responsible for investigating agents considered a security risk. After that, he supervised the development of an internal security department at the FBI when the agency discovered that Robert Hanssen, one of its agents, had spied for the Soviet Union and Russia.

Joe Lewis, who runs the internal corporate investigations unit at Wal-Mart, worked at the FBI for 27 years, serving as acting assistant director for criminal investigations. He works closely with Thomas Gean, chief legal compliance officer, who was the U.S. attorney for the Western District of Arkansas.

In an interview, H. Lee Scott Jr., Wal-Mart's chief executive, said that ''it has reached the point where there are issues that take specialized skills to get to the bottom of.''

Scott conceded that the team has been unusually busy lately. ''You almost have to laugh,'' he said of executives engaging in egregious conduct. ''You can't make this stuff up.''

One of the company's biggest investigations was of a board member and former vice chairman, Thomas Coughlin, whom it accused in 2005 of dipping into company funds to pay for CDs, beer, an all-terrain vehicle, duck-hunting boots and a customized dog kennel. His total theft, Wal-Mart said, was more than $500,000.

As with Roehm and Womack, Wal-Mart spared no detail in its case against Coughlin, who pleaded guilty to federal charges in the case. Investigators documented dozens of improper purchases that included fiber supplements and doughnuts and, in legal filings, described him as a rogue executive committed to defrauding the company.