(Reuters) - A woman armed with a handgun opened fire on Thursday on co-workers at a Maryland distribution center for the Rite Aid drugstore chain, killing three people and wounding three others before taking her own life, the Harford County sheriff said.

The shooting unfolded shortly after the suspect, a temporary employee there, reported for work around 9 a.m. EDT at the warehouse in Perryman, Maryland, about 34 miles (55 km) northeast of Baltimore, Sheriff Jeffrey Gahler said at a news conference.

The sheriff said investigators had not yet determined a motive, but a law enforcement source told Reuters the shooting was believed to have been sparked by a work-related grievance.

The sheriff’s office later identified the killer as Snochia Moseley, 26, a resident of Baltimore County. The fact that the assailant was a woman was unusual given that the overwhelming majority of mass shootings in the United States are committed by men.

Gahler said Thursday’s gun violence began outside the warehouse and then moved inside, but investigators were still piecing together the precise sequence of events.

He said seven people were hit by gunfire and that four, including the woman who opened fire, were killed. The three others were expected to survive, according to the sheriff.

He said the suspect, who was armed with a 9mm Glock pistol and two or three magazines of ammunition, shot herself in the head, and that no shots were fired by law enforcement.

Investigators believe all the victims were fellow Rite Aid employees, but it was not immediately clear whether any were specifically targeted by the suspect, Gahler said.

The Rite Aid center, part of a larger warehouse in an industrial park, has nearly 1,000 employees, company spokesman Pete Strella said. 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-metre) building for possible additional victims, the sheriff’s office said.

“Our prayers are with all those impacted, including our first responders,” Governor Larry Hogan wrote on Twitter. “The State stands ready to offer any support.”

Agents from the Baltimore offices of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives 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, according to Pennsylvania State Police.