Sahel Kazemi "was spinning out of control" when she shot McNair four times as he dozed on a sofa early Saturday, then turned the gun on herself, Police Chief Ronal Serpas said.
Interviews with friends revealed that she was making payments on two cars, her rent was doubling and she suspected the married McNair was having a second affair with another young woman.
She told a friend on Friday that "My life is a ball of s--- and I should end it," Serpas said.
Police earlier had labeled McNair's death a homicide, but waited for further tests and the revelations about Kazemi's personal problems before concluding that she pulled the trigger of a 9 mm semiautomatic pistol in a condominium McNair rented with a friend.
One of the first people to see the bodies in the condominium dialed 9-1-1, but hung up. in Nashville, who called him back and stayed on the line until police arrived.
McNair, 36, a quarterback for the Tennessee Titans most of his career, met Kazemi six months ago at a sports cafe where she was a waitress and his family often ate. She seemed happy and eager to build a life with him, but something went wrong.
"We do know that she was clearly sending a message during the last five to seven days of her life that things were going bad quickly," Serpas said, though there was no indication she told anyone she planned to harm McNair.
Serpas said detectives learned that Kazemi recently found out about another young woman she thought McNair was romantically involved with and had even followed that woman home, though she did not confront her.
Serpas said police believe McNair was asleep when he was killed because there were no defensive wounds. After shooting McNair in the head, Kazemi apparently shot him twice in the chest before shooting him again in the head.
Before shooting herself, she sat next to his body and "tried to stage it so she would fall in his lap," Serpas said. She did, but her body slid to the floor and ended up at McNair's feet. The gun was found underneath her.
Kazemi's family told reporters that the woman was so confident McNair was divorcing his wife of 12 years that she was preparing to sell her furniture and move in with him.
But associate Mike Mu, who has worked with McNair's charitable association for years, said earlier Wednesday that McNair's wife, Mechelle McNair, "didn't know who this girl is." No records of divorce proceedings have surfaced. The McNairs have four children.
Two days before the shooting, police stopped Kazemi driving a Cadillac Escalade sport utility vehicle that McNair had given her for her birthday in May.
According to an arrest affidavit, Kazemi had bloodshot eyes and alcohol on her breath. She refused a breath test and told an officer "she was not drunk, she was high." She was charged with DUI. McNair was with her but not charged. He later made her bail.
Serpas said that even though both her name and McNair's were on the Cadillac's title, she was apparently responsible for making payments. She was also making payments on another car after she couldn't sell it.
Kazemi had no history of violence, but "on the last several days of her life it's obvious that she made some very poor decisions," Serpas said.
Mechelle McNair has not spoken publicly since the shooting. Bishop Joseph W. Walker III of Mount Zion Baptist Church, which the McNairs have attended since moving to Nashville in 1997, said Wednesday that she is doing as well as can be expected.
"Her faith is what's sustaining her now," he said. "We haven't talked about the circumstances of his death. She is processing it in a private way. It's obviously devastating on so many levels."
A memorial service is set for Thursday night in Nashville, with the funeral Saturday in his native Mississippi.
The four-time Pro Bowl quarterback was being remembered Wednesday at the stadium where he played much of his career. The Tennessee Titans opened LP Field for fans to watch video highlights of McNair's 13-year NFL career and look at photos of the quarterback. There was also a book for them to write messages that will be given to the family.
McNair was known as "Air McNair" for his passing prowess at Alcorn State in Mississippi. In 2000, he helped lead the Titans to the Super Bowl, where they ended up a yard short of a touchdown on the last play of the game when they trailed by seven points.
He spent the last two seasons with the Baltimore Ravens before retiring from the NFL last year.