GATINEAU, Que. - The fog of war makes it impossible to prove Capt. Robert Semrau is a murderer, his lawyer argued Wednesday.

Maj. Steve Turner offered his client's defence for the first time in the historic Canadian battlefield court martial that entered its final phase this week, with defence and prosecution lawyers giving their closing addresses.

Turner told the four-member military jury that the prosecution failed to prove its case beyond a reasonable doubt, and asked them to acquit Semrau in the October 2008 death of the heavily wounded Taliban insurgent.

Semrau, 36, is charged with second-degree murder in the death of an unarmed and seriously wounded insurgent in Afghanistan's Helmand province following an intense firefight that pitted Canadian and Afghan forces against Taliban fighters.

When the dust had settled, an insurgent lay on the verge of death. Semrau's four-month trial has been told he pumped two rounds from his rifle into the dying enemy fighter, perhaps in an act of mercy.

Turner said it was a dangerous and chaotic situation, and that the events on the battlefield may have affected the recall of some witnesses.

"It's not like driving home from a car accident after work," Turner argued.

Turner said it simply may not be possible to have a clear picture of what happened that day, and the jury should acquit if the prosecution can't prove essential elements of the case against Semrau beyond a reasonable doubt.

"We just can't throw everything into a blender ... we have to be fairly sure what happened," he said.

"Your role isn't to figure out what's more likely than not."

Turner said forensic evidence was tainted because it could not be properly collected and preserved in the battlefield setting.

"This wasn't something you would see in a CSI episode."

Turner also took aim at the credibility of a prosecution witness, Cpl. Steven Fournier, who testified he heard Semrau make a series of self-incriminating statements after two bullets were pumped into the injured Taliban.

Fournier gave "confusing evidence that was full of conjecture," failed to lead investigators to the correct area where the shooting took place, and offered accounts that were not corroborated by other witnesses, said Turner.

Turner argued that the enemy fighter had suffered catastrophic injuries, and that it would have been unreasonable for Canadian troops to call in a helicopter medical evacuation when there were armed Taliban fighters in the area.

An Afghan National Army captain, who was on the patrol with Semrau, has testified the Taliban fighter was "98 per cent dead" when he was found.

"He certainly might have expired at any moment," Turner told the jury.

But prosecutor Lt.-Col. Mario Leveillee argued that there was no doubt Semrau fired the fatal two rounds, and that the evidence of other witnesses shows the captain made a number of self-incriminating statements.

Leveillee listed several statements attributed to Semrau over the course of his trial:

"I had to help him."

"It was a mercy killing."

"I had to put him out of his misery."

"I will take the fall."

"I will wear it."

"I have killed a man and who knows what will come of that."

Turner told the jury to dismiss those statements because they originated with an unreliable witness and just because the prosecution "homogenizes them together" they shouldn't be taken as fact.

Semrau never testified at the trial, as is his right under military law, and his lawyer presented no evidence.

Leveillee gave his closing statement first, and was not afforded a rebuttal, under the rules of military law.

In a pre-emptive strike, he tried to anticipate a defence that Turner never presented -- that Semrau would seek acquittal on the basis he had committed a mercy killing.

"Mercy killing is simply not a defence," Leveillee told the jury. "It is simply irrelevant."

Leveillee said Semrau, as a trainer of Afghan military forces, did not live up to his legal obligations on the battlefield -- to care for a wounded enemy fighter.

Had the tables been turned, a wounded Canadian soldier would have been entitled to the same level of help, he said.

"An enemy who is injured is no longer an enemy."

Semrau is believed to be the first Canadian soldier charged with murder as a result of a battlefield encounter.

Semrau had a previously spotless record in stints with both the British and Canadian Forces.

A military judge is expected to charge the jury later this week before deliberations begin.