The US army psychiatrist charged in the 2009 Fort Hood shooting rampage will represent himself at his upcoming murder trial, meaning he will question the more than two dozen soldiers he's accused of wounding, a military judge ruled Monday.

Major Nidal Hasan's attorneys will remain on the case but only if he asks for their help, the judge said. Hasan, 42, faces the death penalty or life in prison without parole if convicted of 13 counts of premeditated murder and 32 counts of attempted premeditated murder.

After questioning Hasan for about an hour, the judge, Colonel Tara Osborn, ruled that Hasan was mentally competent to represent himself and understands "the disadvantage of self-representation". She repeatedly urged him to reconsider his request, noting that the lead prosecutor has more than 20 years of experience and that Hasan will be held to the same standards as all attorneys regarding courtroom rules and military law.

"You've made that quite clear," Hasan said after the judge asked if he understood that representing himself was not "a good idea".

Hasan did not elaborate when announcing he would use a "defence of others" strategy, which requires defendants to prove they were protecting other people from imminent danger. Military experts speculated that Hasan may argue he was protecting fellow Muslims in Afghanistan because soldiers were preparing to deploy from the Texas Army post.

Hasan asked Osborn for a three-month delay to prepare his defense. The judge said she would decide that on Tuesday.

Retired staff seargeant Alonzo Lunsford, who was shot seven times during the rampage in November 2009, said on Monday he was upset and angry the judge was allowing Hasan the ability to question the wounded soldiers.

Lunsford said he expected Hasan to try to intimidate them through mind games.

Military experts not involved in the case speculated that Hasan, an American-born Muslim, might try to show he was trying to defend Muslims against US troops in a war he believed was illegal and immoral.

"Even if he feels the US is in an unjustified war, this defendant is not going to be able to show a threat was immediate because these soldiers were on US soil and unarmed," said Jeff Addicott, director of the Center for Terrorism Law at St Mary's university in San Antonio.

Reed Rubinstein, who is representing about 150 Fort Hood victims and their families in a lawsuit alleging negligence by the government, said the wounded soldiers "never had any doubt about why he shot them". But if Hasan tried to use the trial as a platform for his beliefs, "he's making a mockery of the judicial system", Rubinstein said.

At Osborn's request, a doctor testified on Monday about Hasan's physical condition. The doctor said Hasan's paralysis won't have a significant impact during proceedings but that Hasan can only sit for four consecutive hours and has limitations writing. He was paralyzed from the waist down after being shot by police the day of the attack on the Texas army post.

Hasan asked the judge to kick one of his attorneys off the case completely, but she instead said that two of his lawyers would sit at his defense table while the third sits in the courtroom. All will assist him if he asks.

Jury selection is set to start on Wednesday.

Hasan in 2011 cut ties with his previous lead attorney, John Galligan, a civilian who is a former military judge. Galligan said recently that he didn't know why his former client wanted to represent himself.

At a hearing in May, Hasan told Osborn that he wanted to plead guilty. But army rules prohibit a judge from accepting a guilty plea to charges that could result in a death sentence. Osborn also denied his request to plead guilty to lesser murder charges, citing legal issues that could have arisen because his death penalty trial still would have proceeded.

Witnesses have said that after lunch on November 5, 2009, a gunman wearing an Army combat uniform shouted "Allahu Akbar!" – "God is great!" in Arabic – and opened fire in a crowded medical building where deploying soldiers get vaccines and other tests.

Witnesses said the gunman fired rapidly, pausing only to reload, even shooting at some soldiers as they hid under desks and fled the building.

Government reports on investigations after the shooting revealed that Hasan had become a "ticking time bomb" and radical extremist while he was a psychiatrist in training at Walter Reed, where he started in 2004.

The government has also said that Hasan had sent more than a dozen emails starting in December 2008 to Anwar al-Awlaki, a radical US-born Islamic cleric killed in Yemen in 2011. According to the emails released by the FBI, Hasan asked questions indicating he was already thinking about or planning the attack.