Nashville police spokesman Don Aaron identified the woman as Sahel Kazemi, 20, whom he called a friend of McNair's. Police said autopsies were planned for today.
Aaron said McNair's wife, Mechelle, is "very distraught" and that police do not believe she was involved in the shooting.
The bodies were discovered Saturday afternoon by Wayne Neeley, a longtime friend of McNair's who said he rents the condo with McNair.
Aaron said Neeley told authorities he went into the condo and found McNair on the sofa and Kazemi on the floor in the living room. Neeley then called McNair's bodyguard Robert Gaddy, who contacted police.
Police said a witness saw McNair arrive at the condo between 1:30 and 2 a.m. Saturday and that Kazemi's vehicle was already there.
Nashville police arrested Kazemi two days ago on a DUI charge while driving a 2000 Escalade registered to her and McNair. McNair was in the front passenger seat of the Escalade but was allowed to leave by taxi.
McNair, the third overall pick by the Houston Oilers in the 1995 draft, spent 11 seasons with the franchise -- which moved to Tennessee two years later and eventually became the Titans -- before closing out a 13-year career with the Ravens in 2007. He took the Titans to the Super Bowl in the 1999 season, was a co-Most Valuable Player with Peyton Manning in 2003 and was selected to the Pro Bowl three times.
After being traded to the Ravens in 2006, McNair led the team to the AFC North title with a 13-3 record but threw a critical interception on the goal line in a 15-6 loss to the Indianapolis Colts in the divisional playoff.
On the big stage, he will be remembered for a fourth-quarter drive with the Titans that came up a yard short in a 23-16 Super Bowl loss to the St. Louis Rams and for playing winning football in spite of his many injuries.
Reeling at the news, former teammates in Baltimore and Nashville remembered him as a fierce competitor and a strong leader.
Ravens wide receiver Derrick Mason played with McNair in Nashville and in Baltimore.
"Steve was always willing to lend a hand to anyone who needed it," Mason said in a statement issued by the team. "I've known him for 13 years and he was the most selfless, happiest and friendliest person I have known. His family and my family are close, and it is a blow to us all. It is a devastating day. Steve will always have a place in my heart. My family and I are hurting for his family. Our thoughts and prayers are with them."
Cornerback Samari Rolle, who joined the Ravens a year before McNair, was also part of the core group of players who helped the Titans become an AFC power.
"I still can't even believe it," Rolle said. "To lose such a good friend and a good man so soon doesn't make sense. If you were going to draw a football player -- the physical part, the mental part, everything about being a professional -- he is your guy. I can't even wrap my arms around it. It is a sad, sad day. The world lost a great man today."
In 2006, a year after they went 6-10, the Ravens gave up a fourth-round draft pick for McNair, then 33. He replaced the erratic Kyle Boller and started all 16 games that season, winning 13. His costly interception against the Colts in the postseason signaled a change in fortune.
He injured his groin in the 2007 season opener, had back and shoulder problems, and committed 11 turnovers while playing in only six games. In April 2008, before the first minicamp practice of the John Harbaugh coaching era, McNair caught all off guard by announcing his retirement.
Ravens general manager Ozzie Newsome valued McNair's contribution to the team as well as his place in history.
"This is so, so sad," Newsome said in a statement. "We immediately think of his family, his [four] boys. They are all in our thoughts and prayers.
"What we admired most about Steve when we played against him was his competitive spirit, and we were lucky enough to have that with us for two years. He is one of the best players in the NFL over the last 20 years. ... He was a great player, one of the toughest of competitors and a tremendous teammate, a leader on the field and in the locker room, especially to young players."
Speaking on ESPN, former Titans running back Eddie George said McNair "had ice running through his veins. He was so cool under pressure, so calm. He never raised his voice, never lost his composure."
Former Ravens center Mike Flynn called McNair one of the best teammates he ever played with.
"Obviously, everybody saw his ability, but behind closed doors, they couldn't see what a great guy he is," Flynn said. "He was a great guy to play for and play with. It's real sad because he was one of the good guys I've met in 11 years of pro football. I'm stunned and just sad."
Another former Ravens center, Jason Brown, who signed with St. Louis this offseason, learned of McNair's death via text message from Rams guard Jacob Bell. Both had played with McNair.
"It's a very sad day, it really is," Brown said. "You can't shy away from the fact that Steve was a talented human being, and for the two years that I was fortunate and blessed to have played with him in Baltimore, he definitely helped to mature my career by leaps and bounds. ... When he came in in 2006, we did something special that year, and I will never forget the poise that Steve brought to the team that year. My thoughts and prayers go to his family."
Titans owner Bud Adams said in a statement: "We are saddened and shocked to hear the news of Steve McNair's passing today. He was one of the finest players to play for our organization and one of the most beloved players by our fans. He played with unquestioned heart and leadership and led us to places that we had never reached, including our only Super Bowl."
McNair opened a restaurant near the Tennessee State University campus in June. By Saturday night, it was closed and had become a small fan memorial, with flowers, candles and notes placed outside.
A note attached to a small, blue teddy bear read, "We will never forget you, Steve. Once a Titan, always a Titan."
Baltimore Sun reporter Edward Lee and the Associated Press contributed to this article.