Jefferson County, Ala., SWAT team members move down a hallway during an 'active shooter' training exercise at a school in Ketona, Ala., in June. The exercise involved hostage negotiators, a bomb squad and the SWAT team. Associated Press

DENVER—When Michelle Meeker approached a strange man last October at the Colorado nursing home where she worked, the man flashed a gun and forced her into an empty room as she tearfully begged for her life, she said.

Unbeknown to Ms. Meeker, the gunman was a local police officer and the entire episode a drill, arranged by the retirement home's management to prepare employees for an armed-intruder scenario. She filed a federal lawsuit in July against the officer and the facility, located in the small town of Carbondale, alleging she was left so traumatized she had to quit her nursing job.

The lawsuit is one of a raft of recent legal complaints about lifelike "active shooter" exercises, which growing numbers of businesses and schools are using to train for the bedlam of a mass shooting and other violent situations. In some cases, people say they were never told the drills weren't real attacks, while in other instances they maintain that those conducting the exercises got out of hand.

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Michelle Meeker filed a lawsuit against a police officer and retirement home in Carbondale, Colo., alleging an armed-intruder drill left her so traumatized she had to quit her job. for The Wall Street Journal

Robert Baker, executive director of the Heritage Park Care Center, where Ms. Meeker worked, declined to comment on the case, citing the lawsuit, but said the facility conducts routine safety drills.

A lawyer for the Carbondale police department, Thomas Rice, said police believed management had notified employees about the exercise. The officer involved used a fake gun and identified himself after Ms. Meeker became distraught, Mr. Rice said.

"This was a horrifying ordeal," said Paula Greisen, Ms. Meeker's lawyer, who said her client was so overwhelmed that she didn't know whether to believe the officer's assurances that he really was a policeman.

During the past decade, many states began requiring that schools hold lockdown or general-safety drills, apart from natural-disaster preparedness exercises. Since the mass shooting at a Newtown, Conn., elementary school in 2012, five states have passed or amended laws specifically mandating that schools conduct active-shooter drills, according to the National Conference of State Legislatures.