The 2009 killing of Lasantha Wickrematunga, a fierce critic of then-president Mahinda Rajapakse, sparked an international outcry and shone a spotlight on violence against Sri Lanka's media.
Wickrematunga had accused Rajapakse's defence secretary and brother Gotabhaya of taking kickbacks in arms purchases, and was due to testify in court when he was killed.
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A legal source involved with the investigation said the five men were taken into custody over the weekend, initially over the May 2008 abduction of journalist Keith Noyahr, who had criticised military leaders.
"Detectives have found evidence to show that the group which abducted Noyahr was also involved in the assassination of Sunday Leader editor Lasantha Wickrematunga and several other attacks," the source told AFP on condition of anonymity.
Detectives told a magistrate on Sunday that the five accused had operated a military death squad led by a top defence official who has not yet been named.
More senior people are likely to be arrested in the coming months, the source said.
Rajapakse 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.
All deny any wrongdoing and in turn accuse the new government of a political vendetta.
Free Media Movement, a rights group, gave a cautious welcome to the arrests, urging the government to ensure an independent investigation into the allegations and resist "undue influence" from the powerful military.
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President Maithripala Sirisena has publicly criticised police for holding military suspects for long periods, and another group of army intelligence officers arrested in 2015 over the abduction of cartoonist Prageeth Eknaligoda was later given bail and reinstated.
Editor Wickrematunga was killed by a group of men on motorcycles who blocked his vehicle before attacking him with sharp objects.
A retired army intelligence officer was found hanging at his home in October with a note claiming responsibility for the journalist's death.
But police have said they do not believe the claim and are treating the officer's death as a murder.
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