From what I'm gathering, it seems McNair was having an affair with this 20-year-old waitress. They must have had some argument or he was trying to end it or something, and she killed him (he was shot four times - twice in the head, twice in the chest), and she subsequently committed suicide herself. The murder weapon was found underneath her body.
The Nashville police aren't ruling out any situation just yet. But McNair's death has unequivocally been classified as a homicide.
I really feel for McNair's kids and wife. If this is what happened, it's got to make this whole situation that much tougher for them to deal with. McNair was a classy athlete on and off the field, and it's a shame to hear that this is what authorities believe at the moment.