New details have been unveiled in the death of former NFL QB Steve McNair last week in Nashville, and most point to a murder-suicide by his mistress. While officials have yet to confirm the theory because she didn't appear to have a motive, that seems to be the most likely scenario.

After McNair and -- at the time -- an unnamed girl were found dead in an apartment complex in Nashville on Saturday (July 4), police labeled the death a homicide. But as details unfolded, things began to appear obvious. McNair was shot twice in the head and twice in the chest, while 20-year-old Sahel Kazemi was dead from a single gunshot to the head. Under her body was a gun she had purchased less than two days before the killings.

The revelation that it was Kazemi's gun is a strong indication that she was responsible, medical examiner Bruce Levy told the Associated Press.

"If we had known on Sunday about the gun I think we would have been very comfortable in ruling murder-suicide," he said. "I'll be very surprised now if they rule it isn't."

He further explained that Kazemi's lack of motive has made investigators thoroughly explore every possibility, including a scenario that a third party could have staged the scene.

Police said the couple's relationship lacked typical indicators of trouble -- such as concerned family members or police reports and protection orders.

"The thing we always hear is, 'We should have seen this coming,'" he said.

Kazemi's own family said she was very happy, especially with a former football star taking care of her. "She just had it made, you know, (with) this guy taking care of everything," Kazemi's nephew Farzin Abdi said on Monday.

However, according to a report from the Florida Times Union, McNair stood her up just weeks ago in Vegas where the couple were supposed to meet.

Sahel Kazemi's sister told the paper that she expected McNair to leave his wife in the next two or three weeks and eventually marry her. But when she flew to Las Vegas two weeks ago to meet him, McNair never showed up, the sister said.

Early Thursday (July 2), Kazemi was arrested for DUI in a Cadillac Escalade that was registered in both of their names. McNair was with her, but wasn't charged, and police allowed him to leave the scene. He later bailed her out. That night, Kazemi went alone to buy the handgun.

At press time, court records of divorce proceedings had not surfaced.