WASHINGTON - The US military said Tuesday it can't find its copy of a video that shows two employees of the Reuters news agency being killed by Army helicopters in 2007, after a leaked version circulated the Internet and renewed questions about the attack.

"We had no reason to hold the video at (Central Command), nor did the higher headquarters in Iraq," Hanzlik said in an e-mailed statement. "We're attempting to retrieve the video from the unit who did the investigation."

It's the latest twist in a three-year saga that raises questions about the rules of engagement in battle and the safety of journalists sent to cover wars.

Advocates for increased government transparency also have questioned why the military withheld the video from the public, even though Reuters requested a copy through the Freedom of Information Act after watching it in an off-the-record meeting with the military in 2007.

The video includes audio of troops calling to "light 'em up!" and referring to the men as "dead bastards." An internal investigation concluded that the troops had acted appropriately, despite having mistaken the camera equipment for weapons.

"Clearly, it is unflattering to the military, but that is not justification for withholding it," said Steven Aftergood, an expert on government secrecy with the Federation of American Scientists.

The July 12, 2007, attack has been reported before. But Web site Wikileaks.org on Monday posted the video shot from one of the Apache helicopters, putting it on "collateralmurder.com" site.

Military officials said they believed the video was authentic, but that they had to compare the images and audio with their own video before confirming it publicly.

When pressed Tuesday on why the military had not released the video when other documents related to the investigation were made public, officials said they were still looking for it and weren't entirely sure where it was.

The video was taken by the tactical unit that operated the helicopters. The unit has only been identified as a "1st Air Cavalry Brigade," which reported to the Multinational Division in Baghdad.