A 26-year-old woman armed with a handgun opened fire at a Maryland distribution centre for a drugstore chain, killing three people and wounding three others before taking her own life.
The shooting unfolded shortly after the suspect, a temporary employee there, reported for work on Thursday morning at the warehouse in Perryman, Maryland.
The sheriff's office identified the assailant as Snochia Moseley from Baltimore County.
Investigators have not yet determined a motive, but the shooting is believed to have been sparked by a work-related grievance.
A total of seven people were hit by gunfire and four, including the woman who opened fire, were killed. The three others are expected to survive, according to the sheriff.
The Rite Aid center, located among other warehouses in an industrial park, has nearly 1,000 employees. The facility packages pharmaceuticals and other products for delivery to more than 2,500 stores.
Police spent several hours searching the 210,000-square-foot (19,510-square-meter) building for any possible additional victims, the sheriff's office said.
Agents from the Baltimore offices of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) and the FBI also responded, the agencies said.
The shooting occurred a day after a man shot and wounded four people, including a police officer, at a Pennsylvania court building before he was killed by police.