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.