The probe was focused initially on verifying if the images were genuine, after the gossip website TMZ, known for its coverage of celebrities, posted eight grisly photos online.
"We are conducting an investigation into it right now," Captain Tyler Balzer told AFP.
"At this point it's just to really determine the veracity of the photos and what the circumstances depicted in them are, and, if possible, whether we can identify the service members involved," he said.
The TMZ site said it had 41 photos and were told the images were taken in Fallujah, the scene of heavy fighting by Marines nearly ten years ago and where al-Qaeda-linked militants recently took back control.
In the photos, an American Marine appears to be pouring fuel or other flammable liquid on two corpses. Then other pictures show the remains on fire.
In other pictures, a US Marine smiles for the camera as he kneels next to a corpse, and another soldier appears to search the pockets of the clothes on a corpse.
TMZ wrote that it handed over the photos to the US Defense Department.
A Pentagon spokesman claimed the photos do not appear to show a war crime, but the troops in the pictures possibly violated military rules that prohibit the mishandling of remains or inappropriate photos on the battlefield.
"Based on our initial discussions with our legal team, there's been no law of war violations here," Colonel Steven Warren said.
In some cases, torching corpses can be permitted under military rules for "health and hygiene," Warren said.
If military authorities conclude there was wrongdoing, the individuals involved could be prosecuted under the military code even if the incident occurred several years ago, Warren said.
If the troops have since left the military, they would have to be ordered back to duty to face charges, which is extremely rare, he added.
Military commanders have called for tightening up discipline after a series of incidents in the war in Afghanistan.
A team of US Marines faced courts-martial and administrative punishment after a video emerged in 2012 of them urinating on the corpses of Afghan insurgents in Helmand province.
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