DALLAS (CBSDFW.COM) – Federal law enforcement sources confirm the Odessa, Texas mass shooter was denied a gun purchase after being determined “mentally unfit.”

Texas DPS tells CBS 11 the denial came January 14, 2014.

DPS wouldn’t comment further citing its ongoing investigation into Saturday’s shooting in Midland-Odessa that killed seven people and injured 22 others.

The shooter, Seth Ator, 36, was shot and killed by law enforcement officers.

CBS 11 also confirmed the gunman purchased his weapon from a private seller and not a licensed gun dealer, which allowed him to evade a background check.

Seth Ator (credit: Texas Department of Public Safety)

Under federal law, individuals who buy guns from other individuals don’t have to go through such a check.

On Monday, Governor Greg Abbott revealed in a tweet the shooter failed a background check in Texas when he tried to buy a gun and that he didn’t go through a background check when he obtained his weapon.

Retired Dallas ATF agent Tom Crowley said federal investigators are tracking-down the person who sold the AR-style rifle to the gunman.

“It’s all leg work now,” he said.

Crowley said the trace of the gun starts with the manufacturer, then moves to the wholesaler and retailer.

“The retailer will give us the person who signed the form and bought the gun and he’s the last person associated with that gun. So obviously, you go track him down talk to him to say ‘OK, what happened to this gun? What did you do with it’?”

Individuals who sell firearms to other individuals don’t have to keep a paper trail, Crowley said, which could prove challenging to investigators once they find the person who sold the gun to the shooter.

“He could say I forget, it got stolen, or he might cooperate and say I sold it to someone,” Crowley said.

On the criminal background check application, there’s a question asking potential buyers if they’ve ever been adjudicated as a mental defective or if they’ve ever been committed to a mental institution.

If they answer “yes” to either question, they won’t pass a background check.

Other reasons potential gun buyers can fail a background check include if they illegally use or are addicted to marijuana or any depressant, stimulant, narcotic drug, or any other controlled substance or they’re convicted of a felony or a misdemeanor domestic violence charge.

Crowley said whoever sold the rifle to the Odessa gunman, could face criminal charges if they knew he failed a background check.

“They would have to prove that the person selling it or giving it to him knew he was a prohibited person or had tendencies to commit violent acts.”