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.
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.
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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.
"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.