Losses from employee theft cost American retailers $16 billion a year, according to the National Retail Security Survey. That number is almost half the amount lost to shoplifting, and the problem can be very hard to stop. “Once an employee is hired, they have keys and access codes,” said Richard C. Hollinger, a professor of criminology and law at the University of Florida, who compiled the survey. “They’re very hard to deter and very hard to catch.” Sometimes, he said, there is hard evidence, like a video, but that is rare. “Mostly what you have is a cash register that’s a couple hundred dollars short and a couple of people who might be suspects,” he said.

And what is to be done with those suspects? Increasingly over the last 20 years, retailers have turned to internal investigations, often using investigators trained in the same interview and interrogation methods as the police. Their job is to ferret out employee thieves and get them to confess. “An admission makes the case much, much stronger,” Mr. Hollinger said.

But as retailers have used the same methods as the police, they have come under criticism for some of the same unintended results: false confessions.

Retailers don’t think this is a big problem. “Is there a prevalence of false confessions in retail? I’d have to say no,” said Rich Mellor, a recently retired vice president for loss prevention at the National Retail Federation. Loss-prevention people, he said, know that coercion can cost them their jobs and lead to expensive lawsuits.

Because retailers are reluctant to talk to the press, he added, the public generally gets only the former employee’s side of the story, meaning that a lot of cases that seem to be false confessions may not be.

AutoZone did not reply to requests for comment, but lawsuits filed since 2000 have opened a window into how its loss-prevention department operates. Sean Simpson and Charles Moore of the Simpson-Moore law firm in San Diego have represented 10 former AutoZone employees, including Mr. Polston, in lawsuits against the company.

The first was filed in 2001 by Joaquin Robles, an AutoZone employee in San Diego who had helped put store deposits into an armored truck. One day, when the truck arrived at the bank, $820 was missing, and Octavio Jara, a loss-prevention investigator, was sent to speak with him. According to Mr. Robles’s subsequent testimony, Mr. Jara questioned him for nearly three hours, accusing him of stealing the money and telling him that if he would confess, he could keep his job. Otherwise, Mr. Jara would have to call the police.