Police responded to a call at 9.09am BST (14.09pm EST) claiming shots had been fired between Spesutia and Perryman Road, according to sheriffs in the neighbourhood.
The situation is still fluid, with the active shooting still taking place – but so far three have been confirmed dead.
A further two people have been seriously injured in the incident.
The shooting appears to have taken place at or near a drugstore distribution center named Rite Aid, which employs 1,000 people.
CNN reports Rite Aid spokeswoman Susan Henderson Said:”What I understand is the location is secure.”
“The distribution center is where products are received and processed for delivery.
“This is part of a large facility that is a distribution center. The shooting happened adjacent to the primary building.”
There have been reports of multiple victims following whatc the sherriff’s office have called a “horrific situation”.
Gov. Larry Hogan tweeted: “We are closely monitoring the horrific shooting. Our prayers are with all those impacted, including our first responders. The State stands ready to offer any support.”
Harford County Sherrif’s Office tweeted at 2.51pm BST: “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”
Authorities are asking that people avoid the area and drivers have been told to expect traffic and delays in the area.
Flares are set up across Spesutia Road, while part of the road has been sealed off by police cordon.
Witnesses reported seeing emergency services inluding ambulances and polices cars rushing to the scene and confirmed that police were still hunting the shooter. Witnesses also say officers using K-9 firearms teams are searching to locate the shooter.
Witness Colleen Hendrickson said: “I think it’s real for a lot of people right now, and there’s a lot of people still affected by this, [police] are still looking, and it’s been a few hours at this point, or about an hour and a half.
“It’s just insane and it’s still sinking in. It’s still crazy.”
The neighbourhood, a residential community of homes churches and a cemetry as well as a business park, is situated about 30 miles northeast of the city of Baltimore.
The Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) has sent sent agents from Baltimore to investigate, along with agents from the Federal Bureau of Investigation (FBI).
Alexi Scharmann told local station WBAL-TV that her mother, who works at the distribution center, texted her about the shooter. The station reports that the Mrs Scharmann was silent for over an hour before notifying a family member that she was safe.
Local Police will hold a media briefing at 16.45pm BST (11:45am EST)
This is a breaking story. More to follow…