Steve McNair's mistress Sahel Kazemi was busted for DUI two days before she crazily murdered the former NFL star QB and then committed suicide.

The DUI represents one of the circumstances - including Sahel's belief that Steve had another lover - that cops believe led to the grisly killings.

Nashville, Tenn., police have released footage of the incident taken from a dashboard camera. Sahel Kazemi was pulled over for speeding on July 2.

She was then forced to take field sobriety tests, which she failed. Check out the video of Steve McNair's girlfriend getting stopped by the cops ...

Sahel Kazemi DUI Arrest

Cops allowed Steve McNair and another man in the Escalade at the time to leave in a taxi. Steve showed up at the jail later on to post bail for Sahel.

The following day, Kazemi purchased a gun from a Nashville location.

Then, in the early morning hours of July 4, she killed McNair and then herself in a grotesque murder-suicide, the true motives of which are still unclear.

Click to enlarge more Sahel Kazemi pictures, including vacation photos with McNair, who was married with four children, at an undetermined location: