The centre processes products, including pharmaceuticals, for delivery to more than 2500 stores.
The sheriff said the three wounded victims were hospitalised and were expected to survive.
"That's the last thing I heard from her", Scharmann told the station.
People were being asked to avoid the area just outside the city of Perryman. Johns Hopkins Bayview Medical Center spokesperson Monica Stone said in an email Thursday that she was unable to provide details about the patients' conditions.
Colleen Hendrickson, who lives and works in the area, said she was waiting on the bus when someone told her there had been a shooting nearby.
Rite Aid spokeswoman Susan Henderson says authorities are at the scene and the company is cooperating with the investigation.
He said a total of seven people were hit by gunfire and that four, including the woman who opened fire, were killed.
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Chief Newsham said it is more than likely a random attack, but said it was too early in the investigation to say for sure. The motive in the brutal stabbing remains unknown, but authorities do not believe the public is any additional danger.
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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. "I am not joking, and they were running people off the road", he said.
An unidentified assailant opened fire Thursday at a warehouse complex north of Baltimore, killing and wounding multiple people in the latest U.S. mass shooting, authorities said.
When authorities found the suspect she was critically injured on the floor with a gunshot wound to her head, police said.
The FBI described the Aberdeen incident as an "active shooter situation" and said its Baltimore field office was assisting. We have our businesses locked down, so we're closed.
"The situation is still fluid", the Harford County Sheriff's Office wrote on Twitter, adding that officers responded to the incident shortly after 9 a.m. EDT (1300 GMT). "The shooting happened adjacent to the primary building".
Deadly shootings are common in the United States, where the right to bear arms is protected by the USA constitution.
The shooting, described as "horrific" by Maryland Gov. Larry Hogan (R), occurred at a Rite Aid distribution center known as the "Liberty Support Facility".
David Graf, a local businessman, said police "have the whole area blocked off".