''They let us know they'd fire people for going out the fire door, unless there was a fire.'' said Farris Cobb, who was a night supervisor at several Sam's Clubs in Florida. ''They instilled in us they had done it before and they would do it again.''

Mr. Cobb and several other workers interviewed about lock-ins were plaintiffs in lawsuits accusing Wal-Mart of forcing them to work off the clock, for example working several hours without pay after their shifts ended. Wal-Mart says it tells managers never to let employees work off the clock.

Janet Anderson, who was a night supervisor at a Sam's Club in Colorado from 1996 to 2002, said that many of her employees were also airmen stationed at a nearby Air Force base. Their commanders sometimes called the store to order them to report to duty immediately, but she said they often had to wait until a manager arrived around 6 a.m. She said one airman received a reprimand from management for leaving by the fire door to report for duty.

Ms. Anderson also told of a worker who had broken his foot one night while using a cardboard box baler and had to wait four hours for someone to open the door. She said the store's managers had lied to her and the overnight crew, telling them the fire doors could not be physically opened by the workers and that the doors would open automatically when the fire alarm was triggered.

Only after several years as night supervisor did she learn that she could open the fire door from inside, she said, but she was told she faced dismissal if she opened it when there was no fire. One night, she said, she cut her finger badly with a box cutter but dared not go out the fire exit -- waiting until morning to get 13 stitches at a hospital.

The federal government and almost all states do not bar locking in workers so long as they have access to an emergency exit. But several longtime Wal-Mart workers recalled that in the late 1980's and early 1990's, the fire doors of some Wal-Marts were chained shut.

Wal-Mart officials said they cracked down on that practice after an overnight stocker at a store in Savannah, Ga., collapsed and died in 1988. Paramedics could not get into the store soon enough because the employees inside could not open the fire door or front door, and there was no manager with a key.