The man accused of planning a mass shooting at Hartsfield-Jackson Atlanta International Airport will attempt to use an insanity defense, new court documents reveal.

Billy Joe Cagle, 49, appeared in federal court on Thursday morning for a pretrial conference hearing to discuss the logistics of future trial dates.

In court documents requesting a competency evaluation filed earlier this week, Cagle's attorneys say they plan to use an insanity defense. Cagle has already pleaded not guilty to some charges.

Atlanta police arrested Cagle in the Atlanta airport's South Terminal on Oct. 20, after his family alerted the Cartersville Police Department that he threatened to "shoot up" the airport on a FaceTime call while driving, abruptly ending the call after saying, "I'm at the airport, and I'm gonna go rat-a-tat-tat," prosecutors alleged.

Cagle arrived at the airport in a Chevrolet pickup truck that was parked outside the doors to the terminal. When police went to the vehicle, they found an AR-15 with 27 rounds of ammunition. FBI investigators are working to find out how Cagle obtained the weapon.

After receiving the alert with Cagle's photo and description, two officers found the Cartersville man walking inside the terminal. Atlanta Police Chief Darren Schierbaum said investigators believe he was scouting the area before he planned to return to his truck and collect his weapon. 

Authorities estimate that more than 20 people could have been killed if officers hadn't been alerted in time.

Cagle is facing federal charges of attempted violence at an international airport, interstate communications containing threats, and possession of a firearm as a felon. He has remained in custody without bond.

"Mr. Cagle was suffering from a mental, psychological, and/or psychiatric condition that would render Mr. Cagle not guilty of the above counts by reason of insanity," the new court filing reads.

The filing also describes Cagle's mental condition as "episodic, with periods of stability and instability."

In the upcoming weeks, Cagle will be evaluated over his mental state during the alleged incident as well as his current mental state to determine whether he is fit to stand trial.

Defense Attorney Preston Halliburton says that Cagle remains in good spirits and has been cooperating with the legal process.

The difficulty of an insanity defense

It has become harder to succeed with an insanity defense since a federal jury found John Hinckley Jr. not guilty by reason of insanity for shooting President Ronald Reagan in 1981. The verdict stoked public skepticism about insanity pleas, leading to tougher federal and state requirements to reach acquittals.

According to Christopher Slobogin, professor of law and psychiatry at Vanderbilt University, insanity defenses are successful is about a quarter of the cases that reach trial. The defenses are much more common in pre-trial agreements, with nearly seven in 10 insanity acquittals occuring in plea deals.

Some states, including Idaho, Kansas, and Montana, have abolished the use of the defense.

The Associated Press contributed to this report.