Christian Democrat MP Fred Nile has warned of the risks of allowing Muslims to serve in Australia's armed forces.

Reverend Nile says last week's killing spree in the United States in which an army officer shot dead 13 people should cause "all associated with the free world's military forces to feel serious concern".

The NSW MP and former army officer said the Australian Defence Force could already harbour Muslim personnel who may be a threat to their comrades in arms.

"Australians would like to be assured that our defence forces have in place a system of assessment and review which would identify any person whose adherence to any alien ideology might one day override loyalty to mates and loyalty to the Crown," Mr Nile said.

"There is an argument for suggesting that the safety and morale of our troops may warrant a ban on dedicated Muslims joining the armed forces, who may be influenced by Islamic fundamentalism."

Major Nidal Malik Hasan is the only suspect in the mass shooting at the Fort Hood military base in Texas which killed 13 and wounded 30.

Hasan, 39, shot by civilian police to end the rampage, remains in a critical but stable condition at an army hospital in San Antonio, Texas.

Investigators are poring over evidence to determine if Hasan was motivated by Islamist ideology or had snapped under the pressure of his job counselling soldiers traumatised by combat.

According to The New York Times, investigators have tentatively concluded that Hasan was not part of a terrorist plot, but they have not ruled out the possibility he believed he was carrying out an extremist suicide mission.

- AAP