The Army psychiatrist suspected in Thursday's deadly Fort Hood rampage in Texas could get the death penalty if he is convicted of multiple counts of first-degree murder — and military law experts say the evidence against him will be substantial.

American-born Maj. Nidal Malik Hasan has yet to be charged but is expected to face at least 13 counts of murder, one for each of the victims who died, as well as numerous assault and weapons charges in a court-martial.

"Obviously, we're all guessing, but it's reasonable to believe that he will be convicted and sentenced to death," said retired Navy lawyer Philip Cave, now a military crimes defense attorney.

Cave estimated that Hasan, 39, would spend between five and 15 years in the military's court martial system.

"It will be a long charge sheet," military law scholar Richard Rosen told KCBD.com, "one longer than I've ever seen in my life time in the Army."

Though the number of wounded has fluctuated, at least 30, including Hasan, and possibly up to 38 were injured in the mass shooting at the Army base in Killeen.

Army Secretary John McHugh said Friday an investigation is proceeding but no charges have yet been filed against Hasan.

Rosen, a retired colonel who was stationed at Fort Hood for 10 years, called the shooting "tragic and horrible."

"Legal advice is being given at all levels of command right now," Rosen told KCBD.com

Only 10 members of the American military have been put to death with approval from the president since 1951 under the Uniform Code of Military Justice — the armed services' legal system.

The last military execution was the 1961 hanging of Army Pvt. John Bennett for rape. Another defendant, Pvt. Ronald Gray, was scheduled to be executed in December 2008 for multiple murders and rape, but a stay was granted mere days before the execution.

But the massacre at Fort Hood has been called the worst mass shooting ever on an American military base.

"All things being equal, he may well be one of those executions," Cave told FoxNews.com.

SLIDESHOW: Deadly Fort Hood Rampage

Hasan is believed to have methodically and calmly opened fire on his fellow comrades as they filled out medical paperwork and underwent testing at a processing center that handles soldiers coming and going to war.

Around 1:30 p.m. Thursday, witnesses say a man later identified as Hasan jumped up on a desk and shouted the words "Allahu Akbar!" — Arabic for "God is great!" He was armed with at least one semiautomatic pistol capable of firing up to 20 rounds without reloading. He shot about 100 rounds before civilian police officer Kim Munley wounded him with four rounds.

Though his motive remains unclear, speculation has swirled that he was dreading his own imminent deployment to the battlefields in Afghanistan, where he was to continue his work counseling fellow soldiers suffering from post-traumatic stress and other mental turmoil.

Relatives and associates say Hasan was critical of both the wars in Iraq and Afghanistan and did everything he could to avoid being on the frontlines.

No one who knew him, however, expected him to be driven to kill.

The suspected gunman's Palestinian uncle told Fox News that the family was "shocked" by the allegations and had no indication Hasan was capable of such violence.

"He was very quiet, very nice, never been upset, always a smile," Rafiq Ismail told Fox News in Ramallah, the West Bank. "Till now, we did not believe he did it. ... Something happened, made him snap or something."

As a psychiatrist, Hasan had for years listened to other soldiers' tales of war horrors. Cave said if Hasan's lawyers can show that he himself was suffering from post traumatic stress disorder (PTSD) — which can happen to psychiatrists and can be a successful legal strategy — then they might use that in their defense in an attempt to land Hasan a lesser sentence.

But Cave doesn't think Hasan would be sentenced to anything less than life in prison.

Terror charges also could be filed, he said, but only if the government has hard evidence that Hasan was linked to and acting on behalf of an actual terrorist group.

In trying to prove premeditation, Cave expects prosecutors to point to the fact that Hasan had been saying goodbye to friends and giving away most of his belongings, including copies of the Koran, and left several messages for neighbors the morning of the killings.

"Nice knowing you, old friend," Hasan said in a 5 a.m. Thursday voicemail to neighbor Willie Bell. "I'm going to miss you."

Cave said the defense probably would counter that those actions were those of a man about to be sent overseas to war.

Fox News' Reena Ninan and The Associated Press contributed to this report.