Snochia Moseley, 26, of Baltimore County, Maryland, came to work shortly after 6:30 a.m., Harford County Sheriff Jeff Gahler said at a news conference Friday. According to unconfirmed reports from some locals, emergency services scanners say that first responders are "bagging patients".
Gahler says there is no longer any threat at the site of the Thursday morning shooting.
A law-enforcement official with knowledge of the shooting stressed that the number of dead is based on preliminary information.
He said in recent weeks Moseley had become increasingly agitated, and relatives had been concerned for her well being.
Maryland Governor Larry Hogan called it a "horrific shooting".
NBC News and other media outlets, citing law enforcement sources, reported that the assailant was a woman, an unusual circumstance in and of itself as the overwhelming majority of mass shootings in the United States are committed by men. The company employs about 1,000 people at the site.
The woman who shot and killed three people at a warehouse in the U.S. state of Maryland had been employed at the business for less than two weeks.
The Rite Aid centre, located among other warehouses in an industrial park, has almost 1,000 employees, company spokesman Pete Strella said.
Rite Aid spokeswoman Susan Henderson told CNN that the "distribution center is where products are received and processed for delivery".
It appears there was only one weapon, a handgun, that was used and there were no shots fired by responding law-enforcement officers, Gahler said. "The State stands ready to offer any support", Hogan tweeted. She shot herself twice before police arrived, he said _ once with a grazing wound and then with the fatal shot.
Police have officially released the identity of the woman suspected to be the Maryland shooter.
Three other wounded people were being treated at a hospital. Agents from the FBI and the Baltimore office of the Bureau of Alcohol, Tobacco, Firearms and Explosives are also responding to the incident.
Thursday's attack came five months after an Iran-born female animal rights activist gunned down three people before killing herself at YouTube's California headquarters.