The 26 year-old, who has not yet been identified, died Thursday afternoon, several hours after embarking on the handgun rampage in Aberdeen, Md.
Gahler said there was no longer a threat at the site and an investigation was under way. Police blocked off the road outside but were waving in cars driven by people who said they were there to meet up with people who were at the distribution center.
Victims have been transported to trauma centers in Baltimore and across the state border to DE, local CBS affiliate WJZ reported.
Deputies were dispatched at 9:09 a.m., Harford County Sheriff Jeff Gahler said.
Governor Larry Hogan said his office was "closely monitoring the horrific shooting".
Local media said police were using K-9 teams in an apparent effort to locate the shooter.
The Rite Aid centre, part of a larger warehouse in an industrial park, has almost 1,000 employees, company spokesman Pete Strella said. She described it as a support facility adjacent to a larger building.
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. He said the shooter seems to have used a single handgun and that there were "multiple" individuals wounded and killed, but that the situation now seems to be under control.
Mr Carre said the man told him the shooter "just came in in a bad mood this morning".
"Our prayers are with all those impacted, including our first responders", Hogan tweeted.
A coworker injured three people when he opened fire Wednesday at an office complex in Middleton, Wisconsin. Additionally, she said the facility does not have metal detectors and there is only one entrance into that section of the center.
The facility has almost 1,000 employees, company spokesman Pete Strella said, and processes products, including pharmaceuticals, for delivery to more than 2,500 stores. Railroad tracks run alongside a business park.
The attack comes almost three months after a man armed with a shotgun attacked the Capital Gazette newspaper office in Annapolis, killing five people.
The FBI, the Maryland State Police and the Bureau of Alcohol, Tobacco, Firearms and Explosives responded to the scene.
CNN noted, "This marks the third shooting with multiple victims in the country in the span of a day".