Five schools in the Edgewood area were placed on lockdown - meaning students are being kept in the buildings, and visitors are not permitted - as a precaution at the advice of the sheriff's office, the county school system said on its website.
The suspect's name is 37-year-old Radeed Prince.
The Harford Sheriff's Office says officers have responded to Emmorton Business Park and that multiple people were injured in the shooting.
Sheriff Jeffrey Gahler said all the victims are though to have worked at the same company, Advanced Granite Solutions, where the killings took place. They said the suspect is "associated" with the business, sheriff's officials said.
Police said they had been searching for a Dodge Charger in connection with the shooting, but located and eliminated it as a vehicle of interest.
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Three people died at the company's premises in a business park northeast of Baltimore. "Certainly, we consider him armed and risky", he said.
A police chief says the Maryland office park gunman had "beefs" with a man in Wilmington, Delaware, and shot him twice. Prince has been an employee for four months, working as a machine operator, owner Barak Caba told AP in a brief telephone interview.
Prince is believed to have been driving a 2000 black Acadia, with a DE license plate. He says Prince opened fire with a handgun and police are looking for him.
"On the advice of the Harford County Sheriff's Office, these schools are now on modified lockdown due to an incident in the surrounding community", the Harford County school district said. He said it is Harford County's investigation and he could not release any additional information.
"There's an individual out there on the loose who committed one of the most heinous acts we've ever seen in our county".
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