Agents from the Baltimore offices of the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) and the FBI also responded, the agencies said.
Police would not confirm reports that the shooter was a woman, but revealed that the suspect had not been shot by police.
The Rite Aid centre, situated among warehouses in an industrial park, has almost 1000 employees. "The distribution center is where products are received and processed for delivery".
Maryland Gov. Larry Hogan (R) said his office is "closely monitoring" the incident, posting on his official Twitter page that "the State stands ready to offer any support". Two were in stable condition and two who were seriously injured.
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"That's the last thing I heard from her", Scharmann told the station.
Workers at the Rite Aide distribution center in northeastern Maryland described terrifying moments of "crazy" gunfire and people screaming and running in all directions after the shooting.
Police say the gunman who was killed by officers after shooting four people at his Wisconsin workplace on Wednesday is a 43-year-old man who had been employed at WTS Paradigm, a software company, since April 2017. We want to keep our journalism open and accessible and be able to keep providing you with news and analyses from the frontlines of Israel, the Middle East and the Jewish World.
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Sheriff Jeffrey Gahler, of the Harford Sheriff County's Office, said at press conference this afternoon that there were multiple victims, including several fatalities.
"She had reported for her workday as usual, and around 9:00am the shooting began, striking victims both outside the business and inside the facility", Mr Gahler said.
The incident took place around 9 am local time (6.30 pm Indian time) near Aberdeen on the campus of a drugstore chain's distribution centre, which employs close to 1,000 people.
Attacks by women are extremely rare, however, accounting for less than five percent of the total, according to law enforcement officials and academics.
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.
The Harford County Sheriff's Office says the situation is "fluid".
The attack came almost three months after a man armed with a shotgun attacked a newspaper office in Annapolis, Maryland, killing five staff members.