Local police confirmed they were on the scene of the shooting in Emmorton Business Park, Edgewood.
The gunman is still thought to be on the loose, Harford County sheriff’s office said.
Schools in the area, 30 miles from Baltimore, were placed on lockdown as police hunted for the gunman.
Officers have named the 37-year-old suspect as Radee Labeeb Prince.
He is thought to have sped away from the scene in a 2000 black Acadia, with a Delaware licence plate.
The injured two were in a critical condition after being rushed to hospital in Baltimore.
Business owner Steve Chetelat told FOX45 he heard “a lot of people yelling and screaming” as the incident broke out.
Sheriff Jeffrey Gahler said all the victims are though to have worked at the same company, Advanced Granite Solutions, where the killings took place.
He said: “This does appear to be a targeted attack, limited to that business.”
The sheriff said the gunman walked into the business with a handgun and shot five employees just before 9am.
He is thought to have been associated with the company, but police are unsure what that reiationship is.
This story is being updated.