Testimony before the presidential Commission of Inquiry was marred by glaring mistakes in translating questions from English-speaking investigators to ethnic Tamil witnesses, the Centre for Policy Alternatives (CPA) said.
Sittings this week have been affected by errors that could undermine the quality of testimony before the commission, the CPA said, adding that witnesses were photographed by men said to be military intelligence officers.
When one witness was asked in English whether he knew where shells were fired from during fighting, the question was translated into Tamil as: "Can you tell us the camps you were at?" the CPA said.
"Lack of genuine steps at this juncture will severely undermine efforts to arrive at truth, justice, accountability and reconciliation in Sri Lanka," the CPA said in a statement.
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Sri Lanka widened the scope of the commission in July to probe war crimes by both the military as well as the defeated Tamil Tiger rebels. The move was in the wake of international pressure for accountability and the UN setting up an international team to probe Sri Lanka's war record.
Set up in August last year, it has received over 19,470 complaints of missing people, but has so far only heard oral evidence in respect of 939 cases, according to its website.
About 40,000 ethnic Tamil civilians are said to have been killed by government forces in the final months of fighting, a charge Colombo has long denied.
The 1972-2009 conflict claimed 100,000 lives, according to UN estimates.