Police are now looking for a 37-year-old suspect, Radeed Labeeb Prince. Police spokeswoman Stephanie Castellani said the victim at the used vehicle lot is best described as an "associate".
The suspect and the victims were all associated with Advanced Granite Solutions at the Emmorton Business Park in Edgewood.
Investigators believe Prince is associated with the business, but information on the exact relationship was not known immediately. The man said Prince did not get physical with him, but wrote: "I do not want to wait until he will".
To date in 2017 there have been 285 mass shootings - defined as four or more people shot or killed, not including the shooter - in the United States, or almost one per day.
Gahler said Prince was scheduled to work on Wednesday when he walked in and opened fire. Heather Todd said Prince has rented a three-bedroom town house since November 2014 and lives there with his girlfriend. But she said they always ended up paying their rent and the company never had to schedule an eviction.
Harford County Police have identified the suspect in the Emmorton Business Park shooting and have confirmed that he is still at large.
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 is armed and risky.
CNN reported that Prince fled the scene in a black 2008 GMC Acadia with DE license plate PC064273.
Five schools in the Edgewood area were placed on lockdown - meaning students were kept in the buildings, and visitors weren't permitted - in the morning as a precaution at the advice of the sheriff's office, the county school system said on its website. The business park is just south of the Interstate 95 interchange with Maryland 24.