The International Commission of Jurists said its observer on the probe, senior British lawyer Michael Birnbaum, found signs that a 5.
56 calibre bullet was removed and substituted with another type in evidence submitted to local magistrates.
Bullets of 5.56 calibre are used in M-16 rifles, the same type of weapon used by some Sri Lankan special forces, the non-governmental organisation added in a statement.
Seventeen local employees of the French charity Action Contre La Faim (Action Against Hunger) were found shot dead in the charity's offices in the northeastern coastal town of Muttur in August 2006.
The murders still have to be solved. Nordic truce monitors have blamed the Muttur killings on government forces, but the government has denied any role in the massacre.
"Given this new information, the ICJ is calling for the President of Sri Lanka to order renewed, impartial and thorough investigations ...
and to ensure those responsible are prosecuted," the statement said.
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