The 23-year-old asylum seeker who died on Manus Island may have been beaten to death with a piece of timber after G4S guards stormed the detention centre.
Iranian man Reza Berati died and 62 others were injured after the February 17 asylum seeker protest.
A post-mortem examination has since determined Mr Berati was killed by repeated blows to the head, likely by a piece of timber, an unnamed Papua New Guinean police spokesman told Fairfax Media.
A preliminary investigation has also found the death occurred after guards from security contractor G4S, which runs the detention centre, entered the centre and tried to suppress the protest, Fairfax reports.
PNG police and local villagers did not appear to have been involved, according to the preliminary results of the investigation.
The police spokesman said shots were fired as a warning to calm the situation.
"Otherwise, the place would have been burnt to the ground," he said.
Deputy police commissioner Simon Kauba told Fairfax the investigation is continuing and could take all week before being finalised.
