"Please do not try to cover up the investigation. We have noticed lethargy on the part of certain officials," Saman Ratnapriya, a civil society activist told reporters.
"Despite the local and international interest on the case, no proper investigations were done," he said.
Prageeth Ekanligoda, a cartoonist, disappeared in January 2010 while he was working in the campaign of the then presidential candidate Sarath Fonseka who challenged the incumbent Mahinda Rajapaksa.
It was said that Ekanligoda was quizzed by the Army in an eastern military detachment before he disappeared.
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However, police spokesman Ruwan Gunasekera denied any pressure being brought on the CID to suppress the probe.
"The investigations are proceeding. There is no pressure whatsoever," he said.
Sections of the military believes that Eknaligoda had links with the LTTE during the height of the civil war.
"We have to ensure that local mechanisms for justice are well and good. Only then we will be able to stop international investigations", Ratnapriya said referring to action by the UN Human Rights Council for an international probe into alleged human rights abuses during the 2006-2009 conflict.
After Rajapaksa won the 2010 presidential elections, Fonseka was jailed, tried for corruption and treason, and deprived of his political rights.
Maithripala Sirisena, who defeated Rajapaksa in presidential elections in January, had pledged fresh probe into the case after quashing his previous convictions.
In March, the new government also conferred the highest military rank of field marshal on Fonseka after restoring all his political rights.