Sooriyakanthy Thavarajah, 40, a volunteer with the Jaffna branch of the Sri Lankan Red Cross, was taken from his home on Friday and his body was found Sunday, the International Committee of the Red Cross (ICRC) said.
"We are shocked by another senseless and brutal murder of one of our volunteers," the statement quoted Jagath Abeysinghe, head of Sri Lanka Red Cross, as saying.
"The Red Cross and Red Crescent Movement calls upon the Sri Lankan authorities to carry out an immediate and comprehensive investigation into the killing.
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There was no immediate reaction either from the Sri Lankan military or the rebel Tamil Tigers. Each side in the bitter ethnic conflict frequently trade blame over who is responsible for such killings.
The Red Cross also pointed out that two of its local workers were shot dead in June after being abducted from the main railway station in Colombo.
"The Red Cross and Red Crescent Movement reminds all parties to the conflict that they must respect the work