Board members of the New York-based Committee to Protect Journalists (CPJ) met in Washington on Thursday with the Sri Lankan ambassador, Jaliya Wickramasuriya, to lodge their concerns.
The group quoted the ambassador as saying the government was "repulsed" by attacks on the media and "in no way condones or endorses any such attacks."
"While we appreciate the opportunity to convey our concerns directly to the ambassador and his staff, we hoped to emerge from the meeting with assurances of a robust and vigorous investigation into these appalling crimes," said Joel Simon, the group's executive director.
"We were disappointed not to receive any such assurance," he said in a statement.
The rights group charged that assailants who attack the media enjoy impunity and said reporters who wrote critically about the government's campaign against Tamil rebels were subject to harassment.
The committee pressed for further investigation into the death of Lasantha Wickrematunga, editor o