McGuire was killed 16 years after Ellis, in 2007. Williams' fatal shooting occurred in 2008. They were killed with the same gun, Meadows said.
"We identified him through a series of ballistic tests," Meadows said. And police believe Davis may have been involved in two other murders in the area in the past 11 years.
Alfred Glass, a self-employed contractor, was killed Aug. 7, 1998, in a manner strikingly similar to McGuire and Williams, APD homicide detective Vincent Velazquez said.
Glass, 56, was working in his garage when someone shot him twice while his wife and the couple's grandson watched television inside, police said at the time.
A fourth victim, Joseph Williamson, also may have been killed by the same person in 2007, Velazquez said.
"We're looking into the possibility that he's been involved in the murders of [Glass and Williamson]," Meadows said.
According to a 1986 account of the incident in The Atlanta Journal-Constitution, Ellis was shot to death after he allegedly drew a knife on Davis, who complained that his landlord's foster son had stolen money from his room. A loud argument preceded the shooting.
Staff writer Bill Rankin contributed to this article.