WARNING GRAPHIC CONTENT . Pictures were posted on social media 'by U.S.-trained Tikrit One account shows numerous images of beheadings and mutilated bodies . Account holder vows 'revenge ' for massacre of hundreds of cadets by ISIS . Shi'ite militia claimed offensive on Iraqi Special Forces ' would be opportunity for revenge . Images shown to U.S. leaders who say they are evidence of 'war crimes ' Iraqi Army spokesman says investigation will be launched into claims .
WARNING GRAPHIC CONTENT . Pictures were posted on social media 'by U.S.-trained Iraqi Special Forces ' One account shows numerous images of beheadings and mutilated bodies . Account holder vows 'revenge ' for massacre of hundreds of cadets by ISIS . Shi'ite militia claimed offensive on Tikrit would be opportunity for revenge . Images shown to U.S. leaders who say they are evidence of 'war crimes ' Iraqi Army spokesman says investigation will be launched into claims .