GATINEAU, Que. - Moments after allegedly firing two bullets into the chest of a badly injured and disarmed Afghan insurgent, Capt. Robert Semrau told an underling that he did it out of mercy, a military prosecutor says.

Semrau, 36, has pleaded not guilty to unprecedented Canadian charges of a battlefield execution in Helmand province in October 2008. A broad sketch of the prosecutor's case emerged for the first time Wednesday, with Capt. Thomas Fitzgerald laying out for a court martial details that he said sound as if they "come from the pages of a Frederick Forsyth or Ian Fleming novel."

The prosecutor said evidence will include an eyewitness, a nine-minute cell-phone video of the alleged victim taken by an Afghan National Army soldier, military photos, RCMP ballistics evidence of two shell casings and tracer rounds recovered months later, and -- most compellingly -- Semrau's own words.

Fitzgerald said Pte. Steven Fournier, who was with Semrau, will testify that Semrau told him "he could not live with himself if he left a human being and no one should suffer like that."

And a sargeant major from CFB Petawawa will testify that Semrau spoke of a "soldier's pact" during a conversation in January 2009 after he'd been charged: "You do to the enemy what you want or expect him to do to you."

Fitzgerald told the court it took the sargeant major more than a year to come forward with the information.

What the evidence does not include is a body, cause of death, or autopsy report, the prosecutor told the court, although he said that is "not an insurmountable obstacle" to conviction.

Semrau began the day in court standing ramrod straight and answering "Not guilty, your honour," in a soft voice as four charges, including second-degree murder, attempted murder, disgraceful conduct and negligent performance of duty were read out at a Gatineau military complex.

The judge, Lt.-Col. Jean-Guy Perron, told a panel of five officers -- the court martial equivalent of a jury -- in a lengthy address that the proceedings "will likely have to travel to Kandahar" to hear from witnesses there.

Perron warned the panel to "make your decision without sympathy, prejudice or fear."

"You must not use your own ideas about what the law is, or should be."

The case promises to be deeply troublesome for both the military and anyone the least bit sympathetic to the in-theatre plight of soldiers under fire.

The court heard that Semrau was part of a four-member Operation Mentor Liaison Team (OMLT) assigned to an Afghan National Army company. During a 16-kilometre foot patrol they ran into a series of ambushes and called in an Apache helicopter airstrike.

The company soon discovered a badly wounded Afghan insurgent, whom the ANA soldiers disarmed and began verbally and physically abusing, including kicking sand and spitting on him.

The ANA commander allegedly said medical care would not be provided -- "it's in Allah's hands," said Fitzgerald -- and the Canadian OMLT team, led by Semrau, was left to make a decision. They agreed not to call in an evacuation helicopter, although they had that support through the British.

Soon after, a second insurgent was found about 50 metres away, already dead. It was decided to photograph the two insurgents, because they were unidentified and the intelligence could prove worthwhile.

Semrau, Fournier and an Afghan interpreter nicknamed Max returned to the injured man and took the photos. Semrau allegedly then told the other two to "turn around" and that "you do not or should not have to see this."

Fournier and Max will both testify, said Fitzgerald, that they heard two distinct shots or a "double tap," and wheeled around to see Semrau closing the ejection port of his C8 military rifle. Max will testify he turned in time to see Semrau firing the second shot with his assault rifle pointed at the wounded man.

Semrau wasn't charged until two months later as details of the incident began leaking out, the court heard. At that point, a team went back to Helmand province and, with the private's assistance, found the two .56 mm shell casings, but no body.

The court martial, which has already been delayed more than a month by procedural matters, is expected to last into June, said Perron.

Semrau, who had a previously spotless record in stints with both the British and Canadian Forces, was charged late in 2008 and was subsequently released on bail to return to his duties at Canadian Forces Base Petawawa, northwest of Ottawa.