A video by a U.S. military newspaper shows Canadian, American and Afghan soldiers confronting and detaining a convoy of private security contractors whom they believe may have been responsible for the accidental shooting of a Canadian soldier.

The scene was filmed by a video crew from Stars and Stripes hours after Master Cpl. Josh Roberts was shot to death during a joint operation aimed at disrupting insurgent activity in the rugged farming area.

There has been speculation that when members of the security convoy drove by, they opened fire on what they believed were enemy fighters and didn't realize Canadian troops were already in the area engaging the group of about 15 insurgents. Military officials are probing the incident.

In the video, Canadian Major Corey Frederickson explains to the media crew that the soldiers are questioning the contractors, who are local Afghans, because they think it's possible that one of the civilians fired upon Canadians.

"Their normal contact drill is as soon as they get hit with something then it's 360, open up on everything that moves," Frederickson said, referring to the contractors. "We think that's probably what happened, and in the meantime, a coalition soldier got hit. So we're trying to stop as many of them as we can, sort out if any of them know anything and if they'll admit to it."

American Major Kevin Reilly tells the media crew that there are two groups of contractors from two companies: USPI and Compass.

"The Compass convoy is the one we suspect opened up on Canadians," Reilly says.

On the video, the Afghan contractors are heavily armed and some are wearing Afghan police uniforms, although they aren't police. Others are carrying heavy weapons, in contravention of Afghan law.

The video also shows a Canadian soldier questioning one of the contractors.

"When he shot, where was he?" the soldier asks the contractor through a translator.

The translator responds that the man says he wasn't shooting.

"He's lying because before he told me he was shooting," the soldier says.

"Everybody was shooting," the translator says.

"So were you shooting or not," the soldier asks. The contractor shakes his head.

"Unless everybody starts telling the truth, we're arresting all of you, you understand," the soldier says.

As many as 28,000 private soldiers in Afghanistan

Later on in the video, U.S. Army Staff Sgt. David Beau says they "can't get a straight answer. Everybody’s pointing the finger at everybody else."

There are as many as 28,000 private soldiers working in Afghanistan for as many as 73 different companies.

Over the weekend, Brig.-Gen. Denis Thompson defended the use of private security firms.

"Without private security firms, it would be impossible to achieve what we're achieving here," he said. "We just don't have the numbers to do everything.

"As an example, they secure some of our bases. Canadian troops couldn't do their job without the help of private security firms," Thompson said.

Alan Bell, who runs a Canadian private security company that works for both the Canadian and American governments in Afghanistan, said there are regulations in place.

"The only unfortunate thing at the moment, there is a tendency for a lot of companies to not adhere to the regulations and the Afghan government does not have the ability to be able to regulate them the way they should be regulated," he told CBC News Monday.

Bell added that many Afghan security contractors are poorly trained and few take the time to advise either Afghan or NATO soldiers of their operations.

With files from the Canadian Press