Nashville's Chief of Police Ronal Serpas just gave a very thorough and detailed account of the death of Steve McNair and Sahel Kazemi and provided a rather convincing explanation of why investigators have officially ruled it a murder-suicide.
The main points are summed up in this video. Kazemi had a motive, she had the means, and all the physical evidence points to exactly what they thought it would. Kazemi shot McNair in the head, presumably in his sleep, then turned the gun on herself.
Serpas also addressed some of the conspiracy-worthy elements of the case, like our friend Mr. Norfleet (not a suspect), the mysterious 45-minute wait to call the cops (happens all the time), and the seeming illogical nature of the whole crime (murders are never rational.) They're convinced, and there's no reason anyone else shouldn't be too.
UPDATE: I stand corrected. Florio's not buying.
Police confirm murder-suicide in Steve McNair case [Tennessean]
Audio of the 911 call [mp3 file; Tennessean]