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.
Kazemi recently told an associate that her "personal life was all screwed up" and she told a friend on Friday, "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.
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.
EX-QB 'NOT HAPPY' DURING STOP
NASHVILLE, Tenn. -- A police officer tells Steve McNair's girlfriend the ex-NFL star is "not happy" during a DUI stop captured on video two days before the couple died in a murder-suicide.
If Sahel Kazemi was upset about being arrested for drunken driving, it didn't show.
McNair was a passenger in Kazemi's Cadillac Escalade when Nashville police arrested her a week ago.
Police released video of that DUI stop Wednesday. In the video, Kazemi laughed and teased the officer, but also repeatedly asked to have McNair come to the window of the police cruiser where she was sitting.
The officer responded, "He's not happy." McNair, who wasn't charged, left in a cab without talking to her. He later bailed her out.