According to reports filed in a central Sri Lanka court, the grave dates back between 1986 and 1990, the Matale magistrate Chaturika de Silva said yesterday.
The mass grave was found at a construction site of a hospital in Matale district, 140 kilometres from here, in late November last year.
The two reports have been filed by the Kelaniya University Post Graduate Archaeological Institute's professor Raj Somadeva and Matale District Hospital's Judicial Medical Officer Ajith Jayasena.
It was believed that the skeletons from the Matale mass grave were of insurgents from the Marxist party killed during the 1988-89 violence when the current opposition United National Party (UNP) was in power, local media reported.
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The JVP had earlier accused the main opposition UNP rule between 1977-1994 of killing their activists whom they claimed were the victims buried in the mass grave.
The JVP led a bloody rebellion between 1987-1990 against the UNP rule as the Marxist party's opposition to the then President J R Jayawardene's agreement with the then Indian Prime Minister Rajiv Gandhi under the 1987 Indo-Sri Lanka Peace Accord regarding the demobilisation of the LTTE.