Daryl Anthony Birch was found dead in a home in Olney from an apparent gunshot wound to the head, police said.
The manager who was shot by Birch is in guarded condition.
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The shooting happened Tuesday morning on the loading dock of the Kmart located in the 8800 block of Frankford Avenue.
Birch walked into the Kmart right after the store opened with a loaded semi-automatic gun.
He approached his estranged girlfriend, an employee of the store, and an argument broke out between the couple.
After the 35-year-old Kmart manager tried to intervene, Birch shot him three times, in the head, right arm and chest, according to police sources.