GATINEAU, Que. - 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 his chest seconds after two quick shots were fired.

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 in October 2008.

Standing outside the witness box in a military courtroom, 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.

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 ten steps, Fournier said, when two shots were fired "behind me, in quick succession."

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 he was allegedly shot by Semrau.

Fournier testified that neither he nor Semrau administered any 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," said Fournier, and the man's eyes were flickering.

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

Fournier says a captain with the ANA indicated the wounded insurgent's life was in the hands of Allah and ordered that he be left untreated.

"No, no, we leave him. No treatment needed," Fournier testified he heard the ANA captain say.

Moments later, another body was discovered nearby and this insurgent was deemed to be VSA -- or Vital Signs Absent.

Fournier offered to photograph both casualties for identification purposes, which he said the ANA 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.

Fournier testified the wounded man rolled onto his side as they approached, then groaned and rolled onto his back after the photos were taken.

His testimony continues Wednesday.