The US army has dropped an inquiry into whether soldiers posted photographs of dead Iraqis on a website in exchange for access to pornography.
The anonymity of sources made the inquiry difficult, the army says
A preliminary investigation had failed to determine if US soldiers had posted the gruesome pictures and whether these showed actual war dead, officials said.
Colonel Joseph Curtin said the investigation could be reopened if new evidence was presented.
A US Muslim civil rights group condemned the inquiry as insufficient.
Ibrahim Hooper, a spokesman for the Council on American-Islamic relations said the brief investigation "could only serve to further damage America's image and interests throughout the Islamic world".
But Col Curtin said the military was not sweeping the issue under the carpet.
"If the army thinks it's in its interest to investigate something, we will.
"There are multiple challenges here. One is the anonymity of the sources, dates, times, locations, units, anything that is reasonably identifiable that we can work off of.
"Any time new information becomes available that's credible... they potentially could reopen the case," he said.
Col Curtin said issues surrounding how soldiers transmitted pictures from the battleground were being examined.
The website on which the controversial images appear was originally set up for users to trade pornographic pictures of their wives and girlfriends.
Interviewed by the Online Journalism Review of the Annenberg School for Communications last week, the owner of the site said he had offered soldiers free access if they could prove they were members of the military.
Chris Wilson said some sent in pictures of Baghdad traffic signs or of aspects of their life abroad, others sent in pictures of corpses and dismembered bodies.
The pictures are often accompanied by commentaries celebrating the killings but the forum also includes a space for discussion about the war and its purpose.
"This is directly from them [the soldiers]. They can take the digital cameras and take a picture and send it to me, and that's the most raw you can get it. I like to see it from their point of view, and I think it's newsworthy," Mr Wilson told the Online Journalism Review.