An internal Pentagon document obtained by CBC's the fifth estate raises questions about the quality of the investigation conducted by coalition forces into an allegation that as many as 27 civilians were killed in Iraq by a Canadian airstrike.

The Department of National Defence acknowledged last week that an investigation looked into allegations that a Canadian airstrike had killed civilians in northwestern Iraq in January of this year. It was part of an important battle for a key highway near Mosul.

But the department did not disclose the number of alleged casualties, which the document reveals are between six and 27 Iraqis.

"The release of this previously secret [Central Command] document is really very important in our view," says London-based journalist Chris Woods, who leads the Airwars project, which tracks civilian casualties in the war against ISIS in Iraq and Syria.

Airwars coordinated the release of the internal documents with the fifth estate, along with several other media outlets around the world. It was first obtained through an access to information request by the U.S. blog War Is Boring.

The document "details many incidents that I think many of America's allies in the coalition are going to be rather uncomfortable being in the public domain now," says Woods. "And that includes an incident involving Canadian aircraft."

To date, the Canadian military has refused to identify the source of the allegations or provide details about the quality of the investigation, but the document appears to shed new light on both aspects.

The Canadian military continues to say it believes the allegations are not valid.

"The [Canadian Air Force] review identified that there were no substantive grounds to believe that civilians had been killed," Canadian Armed Forces Public Affairs Officer Capt. Kirk Sullivan told the fifth estate in an email.

"Furthermore, subsequent to the allegations, there was no information from the Iraqi Security Forces or government suggesting there may have been civilian casualties."

Source of allegation revealed

The Canadian-linked allegation stems from an airstrike conducted by CF-18s northwest of Mosul on Jan. 21 of this year. The strikes were meant to drive out fighters with the Islamic State in Iraq and Syria, or ISIS, which is also sometimes referred to as ISIL.

For the first time, the internal documents reveal the source of those allegations, the scope of the allegations and the location.

In an area known as "Kisik Junction," the document says, Coalition Special Forces received a report from an "English speaking Pesh soldier" alleging that "between 6 and 27 civilians were killed on 21 Jan as a result of a coalition strike."

"A Canadian strike (CF-18) struck an ISIL sniper/heavy machine gun position on the roof of a building within an ISIL occupied compound which correlates with the alleged report," the document continues.

The original source of the Jan. 21 casualties report is a Kurdish Peshmerga soldier. This is significant, because the Peshmerga are allies in the anti-ISIS coalition and are working with Canadian forces in northern Iraq. (REUTERS)

The original source of the report is significant, because Kurdish Peshmerga soldiers are coalition allies and are working with Canadian forces in northern Iraq, which adds credibility to the allegation.

"It wasn't a question of some civilian or some individual off the street, for lack of a better description, saying, 'I heard this,'" says Stuart Hendin, a Canadian lawyer who teaches international military law to governments and armed forces around the world.

"When one of your allies is saying we have a report of this, it's something that ought to be taken with much more than a grain of salt. It has to be taken a little bit seriously."

The Canadian military will not say if the Kurdish soldier was interviewed as part of the investigation, saying only that it had reviewed photos and video of the incident.

"The source of this allegation heard of these potential casualties through a second hand account," wrote Capt. Sullivan. "The Coalition Headquarters conducted a review of all available reliable imagery and video. The review uncovered no evidence of civilian casualties."

Hendin says that's not good enough.

"You want to see what is the source of his information. What did he see? What did he hear from whom and when?" Hendin says. "There is no indication at all whether there is any follow-up with him."

For more details about the investigation, the Department of National Defence referred CBC News to the United States military, which they say was in charge of looking at the allegations.

The U.S. military is looking into whether the Peshmerga soldier was interviewed.

No duty to investigate: Canada

The internal Pentagon documents also reveal the Canadian military's legal assessment about Canada's duty to investigate the incident.

The U.S. authors of the document note "[Canadian Joint Operations Command Legal Advisor] opinion is that, under the Law of Armed Conflict (LOAC), there are no obligations for the [Canadian Air Force] to conduct an investigation."

This photo features Mansour Barzani, right, the commander of the Peshmerga forces, during the battle with Islamic State militants on the outskirts of Mosul on Jan. 21, 2015. (REUTERS)

The document doesn't say why Canada offered this opinion.

Hendin says he "fundamentally" disagrees with the Canadian military's opinion and says that under international law, there is an expectation that a country will look seriously at any reports involving civilian casualties.

"We have to have enough moral integrity to say we're going to look in the mirror, and if there has been a misdeed, we're going to hold those accountable."

The Department of National Defence defends the comment, and told the fifth estate the allegation was handled appropriately.

"The Law of Armed Conflict, as embodied in the Geneva Conventions and the Additional Protocols to the Geneva Conventions, does not create a positive duty to investigate per se," wrote Capt. Sullivan.

"As to the uncorroborated information about possible civilian casualties resulting from this attack, the information was reported and reviewed by the appropriate coalition organization and CJOC. The source of this allegation heard of these potential casualties through a second hand account. The Minister of National Defence at the time [Rob Nicholson] was informed. The review identified that there were no substantive grounds to believe that civilians had been killed."

Investigations mostly restricted to looking at videos

The internal U.S. military document summarizes 45 separate reports involving dozens of civilians allegedly killed by coalition airstrikes during the anti-ISIS conflict. It reveals new details about what's been reported and how the coalition has investigated those reports.

One allegation details a strike on Nov. 5, 2014, claiming it injured three USAID workers in northeast Syria. The initial review deems the allegation to be "potentially credible" and the investigation continues.

Another points to an Australian airstrike as potentially having injured or killed a woman and child. However, the report goes on to dismiss the allegation because there is "insufficient information to warrant further inquiry."

Of the 45 reports, nine appear to be open investigations, while 21 have been closed after finding what appears to be some evidence that the allegation isn't credible. A further 15 investigations appear to be closed because coalition investigators are unable to determine if an allegation is credible.

However, the bulk of the investigations appear to be restricted to looking at videos and photos of the incidents.

"There is a need, in our view, for the coalition to broaden out and not just rely on video evidence

when it's deciding whether these are realistic allegations or not," says the Airwars project's Woods.

"It needs to be talking to Iraqis, to Syrians, when these allegations emerge, and to find out as much as it can from the ground. It's difficult, but it's not impossible."

the fifth estate will be airing its full investigation into civilian casualties and Canada's role in the war in Iraq and Syria when it launches its fall season next month.