Calvin Munerlyn, 43, died at a local hospital after he was shot in the head Friday, said Michigan State Police Lt. David Kaiser.
The shooter and a second suspect remain at large, Kaiser told CNN on Monday.
Witnesses at the store told police that Munerlyn got into a verbal altercation with a woman because she was not wearing a mask, said Genesee County prosecutor David Leyton. Surveillance video confirms the incident, Leyton said.
Under an executive order from Michigan Gov. Gretchen Whitmer, all retail employees and customers have to wear a mask.
Footage also shows that immediately after the altercation, the woman left in an SUV.
But about 20 minutes later, the SUV returned.
Two men entered the store and one of them yelled at Munerlyn about disrespecting his wife, Leyton said. The other man then shot the security guard.
“This is senseless. Over a mask. Over a mask?” Munerlyn’s cousin, Tina James, told CNN affiliate WJRT. “This is not the way to do things right now. We need to come together.”
Family Dollar did not respond to a request for comment.