Nashville police have revealed that the gun found at the murder scene was purchased by his mistress, Sahel Kazemi, just two days before the shooting.
Investigators believe Kazemi, 20, purchased the pistol from a private vendor, hours after she was busted for drunk driving with McNair in her SUV on Thursday night.
At the scene on the fourth of July, police found the weapon under Kazemi's body in a luxury condo in downtown Nashville, police believe the young woman committed murder-suicide, reports New York Daily News.
The former Titan quarterback was found dead a short six miles from the home he shared with his wife and three children.
McNair's mother, said she could not bear to follow the investigation.
"I don't want to know," Lucille McNair told The Tennessean newspaper. "Because the way I see it, it was the Devil's work and not God's work."
"I'm not mad. I am just grieving over my son," she said. "He was a great son [and] he was special. I just have a hard time talking about it - I get teary-eyed."
It's not clear if McNair's wife, Mechelle, knew about his brazen affair with the Dave and Buster's server.
Kazemi told her sister that McNair was in the process of divorcing his wife.
"That's why she was like, 'OK, now you're divorcing. We can date,'" Sepideh Salmani said. "He told her, it was going to be finished, the whole divorce was going to be done, two weeks from yesterday."
Salmani said her sister believed that she and McNair would move in together once the divorce was final, and eventually marry.
Davidson County's court record show no pending divorce for McNair and his wife Mechelle. McNair's widow has yet to speak publicly about the murders. Police say she is distraught over his death.
In an emotional tribute to his longtime quarterback, Titan's coach Jeff Fisher said, "the Steven McNair I know would want me to say 'I am sorry.'"
"'I am not perfect," Fisher imagined McNair saying. "We all make decisions that are not in the best interest. Please forgive me.'"
A memorial service for McNair will be held in Nashville, where was we-known for his community service. He will be buried in his hometown, Mount Olive, Mississippi on Saturday.