It was in many senses a very modern American story: the retired football legend, the mistress, the perils of fame, and a murder that has gripped the country for days.

But police in Nashville, Tennessee said yesterday that they believe the death of Steve McNair, a quarterback who helped his side muscle into the Super Bowl nine years ago, was the result of an ancient motive: jealousy.

In their first summary of the case since McNair was found dead on Saturday in his Nashville apartment, investigators speculated that a woman half his age found dead next to him, Sahel Kazemi, shot him because she felt her life was unravelling and she suspected him of seeing another woman.

“We can’t put ourselves in the minds of people who do these terribly irrational acts, but we do believe there’s evidence that she was spinning out of control,” Ronal Serpas, Nashville’s police chief said.

Police said Kazemi, 20, shot McNair in his left temple as he slept on his couch in the early hours of Saturday. She then fired three more shots at close range, sat on the couch next to his body and killed herself so that she would fall into his lap, police said. After the shot, her body slid to the floor.

McNair, 36, was among the best football quarterbacks of his era, an African-American excelling in a position typically dominated by whites. He thrilled fans with his ability to elude or knock over defenders and run the ball forward instead of just passing it or handing it off, quarterbacks’ usual moves. He spent seven seasons with the Tennessee Titans, leading them to to the Super Bowl in 2000. He retired from football two years ago after two seasons in Baltimore and returned to the city he had made his home.

Since his death friends have intimated that he found it hard to cope with life after fame. “What people fail to realize is that when you make a transition away from the game – emotionally, physically, mentally, spiritually – you go through something. You change, and you’re constantly searching for something,” former teammate Eddie George told CNN.

Kazemi, known as Jenny to her friends, fled Iran as a teenager with her family. The pair met about six months ago at a middle-brow chain restaurant at a Nashville shopping mall where Kazemi worked as a waitress. Kazemi had just split with her boyfriend and gave McNair, a regular diner, her phone number. “He was one of the nice guys who would talk to you, not like the other athletes,” Brandon Millichamp, Kazemi’s coworker, told the Tennessean newspaper.

McNair took Kazemi on vacations and for her 20th birthday presented her with a luxurious Cadillac Escalde SUV. Whatever he said to her led Kazemi to tell relatives they would ultimately live together and wed. Not being football fans, Kazemi’s family were unimpressed by McNair’s fame and the fact he was already married with four children.

Kazemi’s roommate had decided to move out, leaving her responsible for the entire $1,000 per month rent payment. She had failed to sell her other car and McNair had apparently saddled her with monthly payments on the Cadillac. Kazemi was buried in bills, and with her family living far away in Florida, she felt overwhelmed.

Investigators said Kazemi had also seen another woman leave McNair’s home. She stalked the other woman in an effort to learn her identity but never confronted her, police said. Early last Thursday morning, Kazemi was arrested for drunk driving. McNair was by her side in the Cadillac and was not charged.

Several hours after McNair sprung her from jail, Kazemi’s jealousy and fury reached a boiling point. She paid $100 for a 9mm pistol in a hurried transaction in the restaurant parking lot.

“My life is a ball of shit,” she told a friend the evening before the murder, according to police and “I should just end it”.

Friday night, Kazemi left the restaurant early, about 10pm, and she drove to McNair’s condominium to await his return. McNair had been out on the town, and arrived between 1.30 and 2am. He greeted Kazemi, sat down on the couch for a nap, and never woke up.