(CNN) -- A military appeals court halted the murder case against Maj. Nidal Hasan on Wednesday over a judge's threat to shave the beard the Army psychiatrist grew while awaiting trial in the 2009 Fort Hood killings.
Hasan's court-martial had been scheduled to start Monday at Fort Hood, in Killeen, Texas, where he is accused of killing 13 people and wounding another 32. The presiding judge, Col. Gregory Gross, had threatened to have him forcibly shaved unless he got rid of the beard, which is against Army regulations.
Wednesday's order by the U.S. Court of Appeals for the Armed Forces now means the trial date is unknown. Gross has until August 22 to respond to the appeals court.
Hasan had been expected to enter a plea during a Wednesday hearing, but the proceedings were halted by the appellate court, . Hasan has previously expressed interest in pleading guilty, but military regulations bar a judge from accepting a guilty plea in a capital case.
Hasan is accused of opening fire at the post's processing center, where soldiers were preparing to deploy to Afghanistan and Iraq, in November 2009. The stay came the same day he was expected to enter a plea to the charges against him.
The beard issue first surfaced in June, when Hasan -- who remains in the Army while awaiting trial -- appeared at a June hearing with the facial hair. Gross postponed that hearing, then found Hasan in contempt of court at a July hearing, fined him $1,000 and warned him he would be shaved by force unless he got rid of it.
Hasan faces a possible death sentence if convicted. He was left paralyzed from the waist down in the shooting, when police officers exchanged fire with him.
His lawyers had been seeking a delay of the case until December, but Gross had refused and set Monday as the trial date.
Hasan, a U.S.-born citizen of Palestinian descent, was a licensed psychiatrist who joined the Army in 1997. He had been scheduled to deploy to Afghanistan before the killings, but had been telling his family since 2001 that he wanted to get out of the military.
He is a Muslim who had told his family he had been taunted after the terrorist attacks of September 11, 2001. Investigations that followed the killings found that he had been communicating via e-mail with Anwar al-Awlaki, the Yemeni-American radical cleric killed by a U.S. drone attack in 2011.
An FBI report in July found that a Joint Terrorism Task Force in San Diego that was investigating al-Awlaki passed two of the messages on to another task force in the Washington, D.C., area, where Hasan was living at the time. The report found those e-mails should have been passed to the Pentagon -- but the FBI saw no evidence of terrorist activities in his case, and believed the information in the e-mails was too sensitive to share because visiting extremist websites is not grounds for taking action.