McNair had been dating Saleh Kazemi for several months, and Nashville police spokesman Don Aaron said Sunday that a semiautomatic pistol was found under her body. She was shot in the head.
McNair, who was married with four sons, had a permit to carry a handgun in Tennessee, and he was arrested once before with a 9mm weapon although charges in the case were dropped. Police said they had not yet determined who owned the gun found at the scene.
Investigators weren't looking for a suspect but were questioning friends of the couple as well as Kazemi's ex-boyfriend. They were also waiting for results of drug and other laboratory tests before deciding whether McNair was killed in a lovers' quarrel.
"That's a very important part of the investigation as we work to ultimately classify Miss Kazemi's death," Aaron said.
The details surfacing after McNair's death stand in stark contrast to the public persona he enjoyed during his career.
McNair repeatedly played through serious injuries and pain to win, though he came up a yard short of forcing overtime on the Tennessee Titans' famous drive that came a yard short of forcing overtime in the 2000 Super Bowl.
Generous, he frequently took part in charity work for both the Titans and later the Baltimore Ravens after a 2006 trade. McNair even helped load donated food, water and clothes onto tractor-trailers that he had arranged for Hurricane Katrina victims, and paid for three football camps for children himself this year.
McNair and Kazemi were found dead at a Nashville condominium - which overlooks the Titans stadium - that he rented with his friend Wayne Neeley. Police believe both died early Saturday. Neeley found the bodies hours later, and called a friend, Robert Gaddy, who played at Alcorn State with McNair. Gaddy dialed 911.
"People have certain things that they do in life," Gaddy said. "We don't need to look on the situation at this time (but) on the fact we just lost a great member of society."
The quarterback's agent, Bus Cook, said he had never heard Kazemi's name until news of the shooting broke Saturday. What McNair's wife knew wasn't clear Sunday. Cook said Mechelle McNair was "in and out of it." He said she had no comment after the police called his death a homicide.
"It doesn't make any sense. I don't know what to say," Cook said.
Mechelle was "very upset, very distraught" Sunday, Cook said. She was preparing to finish funeral arrangements Monday.
McNair split his time between Nashville and his farm in Mount Olive, Miss. He recently opened a restaurant near Tennessee State University that was aimed at serving healthy, affordable food to college students.
McNair was also seen so often at Kazemi's apartment that a neighbor thought he lived there.
McNair met Kazemi when his family ate often at the Dave & Buster's restaurant she worked at as a server, and the two began dating in a relationship that included a vacation with parasailing. Photos posted on TMZ.com showed McNair gazing and smiling at the young Kazemi.
"She pretty obviously got mixed up way over her head with folks," said Reagan Howard, a neighbor of Kazemi's.
A man who answered the door at a house in the Jacksonville, Fla., suburb of Orange Park said it was the home of Kazemi's family, but said her relatives did not want to comment.
"We don't have anything to say, please leave us alone," he said.
The victim's sister, Soheyla Kazemi, told the Florida Times-Union in Jacksonville that the young woman had expected McNair to get a divorce. "She said they were planning to get married."
Kazemi often was dropped off by limousine in the early morning hours and recently went from driving a Kia to a 2007 Cadillac Escalade registered to both herself and McNair. Her niece told The Tennessean that Kazemi thought McNair was divorcing his wife of 12 years soon.
Nashville courts had no record of a McNair divorce case, but a home he owned in Nashville is on the market for $3 million.
The real estate agent declined to comment. Her online listing for property described it as a "gigantic house" of more than 14,000 square feet and photos showed a pool, home theater, baby grand piano and ornate furnishings throughout.
McNair and Kazemi were together Thursday night when she was pulled over driving that Escalade. She was arrested on a DUI charges, and he was allowed to leave in a taxi even though he was charged with drunken driving in 2007 when his brother-in-law was stopped for DUI while driving McNair's pickup truck.
McNair led the Titans to the 2000 Super Bowl, which they lost 23-16 to the St. Louis Rams despite his 87-yard drive in the final minute and 48 seconds. He was co-MVP of the NFL with Colts quarterback Peyton Manning in 2003.
Manning said in a statement Sunday that he had some great battles with the quarterback.
"Sharing the NFL MVP honor with him in 2003 was special because of what a great football player he was," Manning said. "I had the opportunity to play in a couple of Pro Bowls with him, and the time spent with him in Hawaii I'll never forget. I'll truly miss him."
The Titans drafted Vince Young in 2006 to replace McNair, who had mentored him since he was a teenager. They never played together but did play against each other that year.
"He was like a father to me. I hear his advice in my head with everything I do. Life will be very different without him," Young said in a statement Sunday.
McNair grew up in Mount Olive, Miss., and became a football star at Alcorn State, the Division I-AA school in his home state as he dominated the Southwestern Athletic Conference. He became a Heisman Trophy contender as reporters flocked to little Lorman to watch the man known as "Air McNair."
He still holds the Division I-AA (now known as Football Championship Subdivision) records for career yards passing (14,496) and total offense (16,823). McNair was drafted in 1995 by the Houston Oilers, who eventually became the Titans.
Picked four times for the Pro Bowl, McNair finished with 31,304 yards passing and 174 touchdowns. He led both the Titans and Ravens to playoff berths, including two AFC championship game appearances with Tennessee. Injuries finally led to his retirement after the 2007 season
Besides his wife, McNair is survived his sons Junior, Steven, Tyler and Trenton.
Associated Press writers contributing to this report include: Ron Word in Jacksonville, Fla.; and Travis Loller, Lucas L. Johnson II and Joe Edwards in Nashville.