US police added that officers did not fire any shots when they arrived at the scene in Aberdeen, about 30 miles northeast of Baltimore.
Police have not confirmed that the suspect, who is under armed guard in hospital, is a woman.
Harford County Sheriff Jeffrey R Gahler said there was no longer any threat to the public after the incident in the Perryman area.
He said there were multiple fatalities and multiple injuries but that authorities were taking care in the preliminary investigation to avoid giving incorrect information.
A law enforcement official has said that three people were killed in the shooting, based on preliminary information.
We can confirm there was a shooting in the area of Spesutia Road and Perryman Road. Multiple victims. The situation is still fluid. Please avoid the area. Media staging area still TBD.
The shooting occurred at a Rite Aid support facility, a pharmacy-like company where about 1,000 people work.
Mr Gahler said the call about shots fired came in from the Rite Aid distribution centre at 9.06am local time (1pm GMT), and deputies and other officers were on the scene in just over five minutes.
Susan Henderson, a spokeswoman for the company, said What I understand is the location is secure.
The distribution centre is where products are received and processed for delivery.
We are closely monitoring the horrific shooting in Aberdeen. Our prayers are with all those impacted, including our first responders. The State stands ready to offer any support. https//t.co/fzugpo8C1Z
This is part of a large facility that is a distribution centre. The shooting happened adjacent to the primary building.
Before the incident ended, the FBI’s Baltimore field office tweeted that it was responding and assisting the sheriff’s office with the situation.
The Baltimore field division of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives tweeted that its special agents were responding too.
Maryland Governor Larry Hogan said We are closely monitoring the horrific shooting in Aberdeen. Our prayers are with all those impacted, including our first responders.
The State stands ready to offer any support.
News Source SkyNews