The New York-based Human Rights Watch (HRW) said it had examined over 200 photos taken on the front lines in early 2009 by a soldier from the Sri Lankan Air Mobile Brigade and had new witness accounts of troops shelling civilians.
Among the photos was a set of images which first showed a political activist with the Liberation Tigers of Tamil Eelam (LTTE) captured by the security forces and then showed him dead with a head injury.
"While Human Rights Watch cannot conclusively determine that the man was summarily executed in custody, the available evidence indicates that a full investigation is warranted," the rights group said.
It accused Sri Lanka of consistently failing to probe allegations of rights abuses and urged the international community to insist on an independent investigation as demanded by the United Nations and many others.
"Witnesses told HRW about three other incidents in late April and early May 2009 of government forces shelling civilians, mainly women and childr