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.
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.
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.”