A PNG police spokesman, Dominic Kakas, said the investigation was currently under way and that it was an "ongoing case".

Mr Kakas said no one had been charged, nor interviewed, for the murder of the 23-year-old asylum seeker. "It's got to be soon," he said.

On Monday, Immigration Minister Scott Morrison refused to comment on the progress of the Barati murder investigation, telling reporters in Canberra: "I have no further information to provide you on that than what is currently publicly available and as I have provided previously."

The Robert Cornall report into violence at the detention centre found a Salvation Army worker led the attack on Barati.

In May, headway was being made with the investigation when PNG police asked security firm G4S for a full list of employees working on Manus Island in mid-February, as well as photos of them.

At the same time, Mr Kakas said one of the four chief suspects in the killing of Iranian asylum seeker Reza Barati had fled Manus Island and his whereabouts were unknown.

Since the violence in February, not one asylum seeker has been taken to Manus Island, it was revealed during the Senate-run Manus inquiry last week.

Tensions rose between security staff and detainees in the Manus Island detention centre on Monday night, resulting in two asylum seekers being confined by Wilson security staff. It is believed the two men were taken from their compound by security after they became aggressive, complaining about their treatment on the island.

Correction

The story "Five months after detainee killed, no suspect charged" (July 16) should not have said the government report into violence at the Manus Island detention centre in February found a Salvation Army worker was responsible for the final blow to Reza Barati's skull with a rock. The report said a Salvation Army worker "led" the attack on Barati.