Police have already arrested six military intelligence officers in connection with a 2008 attack.
Sirisena, who has been criticised for failing to establish credible investigations into war-era abuses, said he would not stand by murderers but would defend "war heroes" who helped crush the Tamil Tiger rebels in 2009.
"Those who killed journalists, sportsmen and others will not be protected," he said.
"Whether they are in the military or the police is immaterial."
His remarks will likely be seen by senior police investigators as a green light to arrest several prominent establishment figures over the 2009 assassination of respected newspaper editor Lasantha Wickrematunga.
His murder heightened global condemnation of former president Mahinda Rajapakse's regime, which had already been accused of abuses against journalists, activists and the Tamil minority population in the dying days of the war.
Police have already arrested six military intelligence officers in connection with a 2008 attack on another editor, and say the same death squad was also responsible for killing Wickrematunga in January 2009.
Rajapakse's defence secretary brother, Gotabhaya, has been implicated by his then army chief Sarath Fonseka of leading the group.
Gotabhaya and Fonseka have been at loggerheads since the end of the war and have often accused each other of wrongdoing.
Rajapakse, whose tenure ended in 2015, and several members of his family are under investigation for large-scale financial fraud and murder during his 10 years as president, in which 17 journalists and media workers were killed.
A retired army intelligence officer was found hanging at his home in October with a note claiming responsibility for Wickrematunga's death.
But police have said they do not believe the claim and are treating the officer's death as a murder.
Sirisena has previously rejected calls for an international trial into war-era crimes, emphatically stating he would never prosecute his own troops.
The president stunned his own coalition allies in October when he berated police for holding intelligence officers for long periods in custody in connection with the 2010 abduction and disappearance of a cartoonist.