The Papua New Guineans charged with wilful murder in the case are expected to face a court hearing this week on Manus.
But for the two Australian expat workers, there is little sign they will be returned to face the justice process.
The skull of Iranian asylum seeker Reza Barati was fatally smashed in a brutal confrontation on February the 17th, 2014.
It was a confrontation that left dozens of detainees and staff injured.
About 10 workers and guards allegedly assaulted Mr Barati in Australia's Manus Island detention centre.
Manus provincial police commander Alex Ndrasal says shortly afterwards, two expats allegedly involved left Manus and Papua New Guinea.
"Of course, there were two implicated. Unfortunately, we cannot bring them back, but that is the point that we are still struggling on, having to get these two guys back here to face their charges as well. We already arrested the two nationals."
Two Manus Islanders employed by the then centre operator, G4S, and the Salvation Army face wilful murder trials.
The police investigation found, "Two expatriate security guards ... also assisted in beating the bleeding deceased whilst he was lying on the balcony floor."
Eighteen months on, Police Commander Ndrasal is frustrated.
"We know the first names of these two expats. The second name, it is a struggling issue we are trying our best to find out what is their second name. In relation to Reza Barati, probably the previous management did not cooperate very well with the local police to speed it up, and that is the whole reason why we cannot bring them back immediately."
G4S says it has not had a request from police regarding the expats.
It says it supports what it calls "the full and proper application of PNG law" and "continues to cooperate fully with the PNG police investigation".
Papua New Guinea says Australia has repeatedly breached its sovereignty by removing suspects in several cases on Manus before police could interview them.
Prime Minister Peter O'Neill has raised the issue directly with Australia.
"How would you feel if we treated Papua New Guineans the same way when they caused offences in Australia, and we evacuated them on the same day? We have to respect each other's laws and deal with it. We do the same. We respect Australian laws when our own citizens cause issues down there, so we expect Australians to do the same."
Rape and drink-driving cases on Manus are stalled due to the absence of expats allegedly involved.
The murder trial of the two locals in the Reza Barati case is listed for this week on Manus, but it is expected to be deferred.
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