The 52-year-old former military intelligence officer was found hanging at his home in central Sri Lanka five days ago and was buried shortly afterwards, the senior police source said.
But police now suspect the suicide note found in his pocket at the time of his death may not be genuine and want to determine his link to any criminal activity that could have led to his murder.
"We are looking at his fingerprints to see if he had been involved in any criminal activity."
Investigators have begun checking his phone records and bank accounts since his retirement nine years ago.
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Last month authorities reopened the investigation into the January 2009 killing of editor Lasantha Wickrematunga, a fierce critic of the then-strongman president Mahinda Rajapakse.
A serving military intelligence officer is already in custody in connection with Wickrematunga's death, which sparked an international outcry and shone a spotlight on violence against the media.
Wickrematunga was the chief editor of the Sunday Leader newspaper which had accused Rajapakse's then-powerful defence secretary brother Gotabhaya of kickbacks in arms purchases, including an aircraft deal.
Last year a former minister in Rajapakse's government accused Gotabhaya of running a death squad and ordering the hit against the editor.
The newspaper was highly critical of rights abuses by troops against ethnic minority Tamils during the island's long-running separatist conflict that ended in 2009.
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