GATINEAU, Que. - "It's OK, it was me."

A key prosecution witness has testified those calmly spoken words by Capt. Robert Semrau helped reassure him that two bursts of rifle fire came from Semrau's rifle, not the enemy.

Semrau, 36, is charged with second-degree murder in the alleged battlefield execution of a badly wounded and disarmed Taliban insurgent in Afghanistan in October 2008.

Cpl. Steven Fournier, who served as a private in the four-man Operational Mentor and Liaison Team led by Semrau, provided a second day of dramatic testimony at the unprecedented court martial Wednesday.

Fournier says he and an Afghan interpreter were walking away from the wounded insurgent -- having been told by his captain that "we don't have to see this" -- when he heard two quick bursts of gunfire.

He testified he spun around looking for enemy threats, only to see Semrau standing over the insurgent with the muzzle of his C8 assault rifle only a metre from the man's chest.

"It didn't make sense to me why he would be shooting in that direction," Fournier, a beefy army driver who was on his first military tour, told the court martial.

Fournier testified Semrau then calmly stated he hadn't expected the two rounds "to go through him like that," and that Semrau said the rounds had ricocheted off the ground.

Fournier and the interpreter are the critical witnesses for military prosecutors seeking conviction in a murder case that has no body and no cause of death. The Afghan insurgent, although seen by the court to be badly injured and unmoving in a cell phone video taken by Afghan National Army soldiers, was never recovered or identified.

Semrau, a Petawawa, Ont.-based father of two young daughters who served in the British military before joining the Canadian Forces, has pleaded not guilty to four charges.

Fournier had already told the court martial that a captain with the ANA had ordered that the insurgent not be treated and be left behind. The man was covered in a blanket but his eyes were fluttering, he was groaning and would occasionally role from his side to his back, then return, Fournier testified.

Late Wednesday, under cross-examination, Fournier acknowledged that the insurgent no longer appeared to be bleeding and -- given his gaping wounds -- he believed the man had bled out. Asked by defence counsel whether he felt the casualty was beyond saving, even with medical assistance, Fournier responded: "I assumed that, yes sir."

Fournier earlier testified that even after hearing the shots and seeing Semrau poised over the insurgent, he still didn't quite grasp what had taken place.

As they walked away from the body, he testified, Semrau made it perfectly clear. Fournier testified that Semrau told him that he "felt it was necessary."

"He felt it was the humane thing to do, he couldn't live with himself if he left an injured insurgent, an injured human, behind," and then described the act as a "mercy killing."

Fournier said he was in a state of shock and did not ask any questions.

Upon returning to the main group of Afghan soldiers, Fournier said ANA Capt. Shafiqullah marched up to Semrau "clearly upset" and shouted something in broken English about "why didn't we leave him alone?"

Fournier says Semrau then gathered the other two Canadian soldiers in their mentor team and explained what had just happened.

According to Fournier, Semrau confessed in a lowered voice his actions to Warrant Officer Merlin Longaphie and Cpl. Tony Haraszta.

"He told us he fired the shots," Fournier testified. "That he felt it was necessary, that he couldn't live with himself if he left an injured human. . . . He hoped that anyone would do the same to anyone else, even him," and again called the shooting a "mercy kill."

Longaphie has already testified that he did not speak with Semrau after the incident, did not witness any shooting and did not get a close look at the insurgent to see if he was dead or alive.

That wasn't the only contradictory testimony that emerged from Fournier.

At the end of the day-long mission, after marching at least 22 kilometres, Fournier said Semrau was "almost immediately" approached by Maj. Steven Nolan -- his commanding officer -- when the team returned to its camp for the evening.

Fournier said he observed Nolan and Semrau walk away behind a vehicle and speak for about five minutes before Semrau returned.

Nolan has testified he discussed the general operation with Semrau and others during a 45-minute debriefing later that evening but was not told anything of the alleged battlefield execution.

Fournier said he told no one about the incident for more than two months, including during a leave to his home in Thunder Bay, Ont. But when he was approached in Afghanistan by Chief Warrant Officer David Fisher in late December 2008 and ordered to relate what had happened on Oct. 19 with "no lies," Fournier testified he was "quite relieved."

He said Fisher, the senior non-commissioned officer overseeing the OMLT groups, seemed aware of what had taken place and Fournier no longer felt the burden of being the individual to "rat out" Semrau.

The Canadian Forces' National Investigation Service interviewed Fournier within hours of his talk with Fisher, and Fournier later helped the NIS find the exact location of the alleged shooting in Helmand, where two shell casings and two bullets were collected using a metal detector.

The defence cross-examination of Fournier continues.