Capt. Robert Semrau, shown in October 2008 preparing to go to battle, has been charged with second-degree murder in connection with the shooting death of an Afghan insurgent. (CBC)

The key prosecution witness at an unprecedented court martial says he saw a Canadian Forces captain standing over a captured Afghan insurgent with his rifle pointed at the insurgent’s chest seconds after two quick shots were fired.

Army Capt. Robert Semrau, 36, is charged with second-degree murder for the alleged battlefield execution — or mercy killing — of a severely wounded and disarmed Taliban fighter in October 2008.

Standing outside the witness box in a military courtroom in Gatineau, Que., Cpl. Steven Fournier demonstrated Semrau's stance with his C8 assault rifle barrel "at a perfect 45-degree angle" toward the wounded insurgent.

"The muzzle was within a metre of the man on the ground," Fournier testified.

Fournier, the most junior member of Semrau's four-man operational mentor and liaison team, or OMLT, is the crucial witness in a bizarre murder trial that includes video footage of the alleged victim but no body and no confirmed cause of death.

Fournier testified Tuesday that Semrau told him and an Afghan interpreter to leave him alone with the injured insurgent: "We can head back — that we don't have to see this."

They had gone no more than 10 steps, Fournier said, when two shots were fired "behind me, in quick succession."

Body never recovered

He spun around, looking for incoming fire, only to see Semrau standing almost directly over the insurgent and closing the ejection port on his C8.

The body of the insurgent has never been recovered, although the court martial was shown a cellphone video of the limp, unmoving casualty taken by an Afghan National Army soldier shortly before Semrau is alleged to have shot the insurgent.

Fournier testified that neither he nor Semrau administered first aid or checked the wounded insurgent for vital signs. But he said he saw that the man's leg was all but severed with one foot "rotated 180 degrees the wrong way," he had another deep shrapnel wound above the knee and an open laceration on his stomach.

"No blood was coming from any of the wounds" and the man's eyes were flickering, Fournier said.

Major Taliban sweep

Semrau, Fournier and two other Canadian soldiers formed a mentor and liaison team that was working with the Afghan army during a major operation to sweep out insurgents in Helmand Province.

Fournier said after they discovered the wounded insurgent, another was discovered nearby and this insurgent was deemed to be VSA — vital signs absent.

Fournier offered to photograph both casualties for identification purposes, which he said the Afghan National Army reluctantly agreed to.

Fournier, Semrau and an Afghan interpreter nicknamed Max returned to the first insurgent, who by then had been left alone covered in a blanket.

The corporal’s testimony continues Wednesday.