Lasantha Wickrematunga, a prominent critic of the former administration, was stabbed days before he was due to testify in a corruption case involving the then defence minister Gotabhaya Rajapakse.
The killing sparked an international outcry and shone a light on human rights violations in Sri Lanka under former president Mahinda Rajapakse, Gotabhaya's brother.
"What about bringing the perpetrators to book," he said, adding there was a "sense of hopelessness" over the government's handling of the case.
"Justice needs to be done not as a favour. Justice needs to be done to prevent repetition."
Wickrematunga had accused the former defence secretary of taking kickbacks in arms procurements, including the purchase of second-hand MiG jet fighters.
Gotabhaya Rajapakse has been accused of giving orders to a shadowy military outfit allegedly involved in murdering journalists and political dissidents during Sri Lanka's long- running civil war, an allegation he denies.
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