US military prosecutors concluded their case Tuesday against Major Nidal Hasan, who faces the death penalty for gunning down 13 of his comrades at a Texas military base in 2009.

Hasan, who has conducted his own defense and does not deny the killings, was to have an opportunity to sum up on Wednesday and a verdict could come later this week.

The 42-year-old Virginia-born army psychiatrist has said that he carried out the shooting spree at the Fort Hood army base in defense of his Islamist ideals.

The case is being tried at Fort Hood in front of a jury of 13 army officers.

His attack raised fears that the United States could face a wave of so-called "lone wolf" killers, inspired by Al-Qaeda but not directly under the extremist group's control.

"I was defending my religion," Hasan said in a letter to AFP, arguing that the United States was wrong to invade Muslim countries.

"It is one thing for the United States to say 'We don't want Sharia law to govern us,' but it is not acceptable to have a foreign policy that tries to replace Shariah law with a more secular form of government."

Under US military law a full trial must be held in a death penalty case, even if the defendant wants to plead guilty.

But Hasan chose to defend himself and contested virtually none of the evidence presented during the two-week court martial.

At the outset, Hasan declared: "The evidence will show that I am the shooter."

He told the court he had switched sides in the war on terror in order to wage a battle against US soldiers he believed would attack Muslims in Iraq and Afghanistan.

His court-appointed legal team has argued that, far from seeking to defend himself, the major is actively pursuing the death penalty.

The defense team attempted to intervene on the second day of the trial, asking the court to allow them to distance themselves from the case or take over Hasan's defense outright.

Their request was rebuffed, and if prosecutors are judged to have proved that the attack was premeditated then the death penalty remains on the table.

Testimony from more than 90 witnesses recounted how Hasan opened fire on soldiers at a crowded waiting station where soldiers undergo routine medical tests prior to deployment.

Hasan himself had recently learned he would be deployed to Afghanistan.

Witnesses described a macabre and chaotic scene in which dozens of soldiers were caught off guard.

When the shooter shouted "Allahu akbar" -- Arabic for God is great -- and opened fire, many believed it was a training exercise.

Several described the methodical nature of Hasan's attack, picking strategic points that gave firing lines to both exits of the facility.

It allowed the major to fire at a rapid pace, hitting some victims multiple times. One of the dead, Specialist Frederick Z. Greene was hit 12 times while charging Hasan.

The shooting ended after civilian police confronted Hasan outside the building.

Former Fort Hood police officer Kimberly Munley was the first police officer on the scene and opened fire on with the shooter.

Hasan charged Munley and shot her three times. Fellow officer Sergeant Mark Todd then arrived and opened fire on Hasan.

Todd hit Hasan multiple times, leaving the major paralyzed from the chest down.