The statements were handed to police on the island by the Sydney lawyer who obtained them before he was deported from Manus Island on Friday, having been shut out of the detention centre in defiance of a court order.

The deportation thwarted a planned meeting between the barrister, Jay Williams, key investigators and the asylum seekers, who until now have been too scared to give authorities their account of the slaying of Mr Barati, and injuries to more than 60 other detainees on March 17.

But Mr Williams gave the investigators a letter to pass on to the witnesses. The statements are from three asylum seekers, including his roommate, who say they saw how Mr Barati died; one who was attacked, then shot in the buttocks when he fled and bashed again; and one whose throat was slashed while he says two men pinned his arms.

Fairfax Media believes that one of the witnesses says Papua New Guinea guards, PNG locals and expatriate Australians were involved, and that he can identify most of them by face or name.

The potential breakthrough came as police revealed that two Australian employees of the security contractors G4S were wanted for questioning over the violence, which came after asylum seekers say security guards and others broke into the centre.

''They are expatriates,'' a police source said. ''But they returned to Australia soon after the incident. We are recommending that once we have completed our investigation of these two people, we will ask that they be extradited back to PNG to face possible criminal charges.''

The PNG investigators know only the two men's first names. Fairfax Media has chosen not to publish the names.

The source, citing a police report from March 26, compiled by Chief Superintendent Anthon Billie, acting divisional commander for the islands region of PNG, said that ''a major obstacle'' to the investigation had been G4S and its employees' refusal to co-operate with the PNG police.

The company and its workers have insisted on speaking instead to the Australian Federal Police, the source said, adding that PNG had requested that two AFP officers be sent to Manus Island to help the investigation.

The AFP had not responded by deadline to Fairfax Media's questions.

No reason was given for Mr Williams' deportation on Friday, which ignored an order from the PNG judge investigating human rights at the centre who allowed Mr Williams into the centre before a new inquiry was due to open on Monday.

When Mr Williams asked for a copy of the deportation order after arriving in Port Moresby, none was produced and he was asked to leave the country voluntarily. He declined, and plans to appear at the inquiry on Monday and to institute contempt proceedings against the government.

The lawyer had been allowed into the centre between 10am and 4pm each day this week by Justice David Cannings, who instituted a constitutional inquiry into human rights at the centre after the violence.

When the PNG government, acting after consultations with Australia, moved to shut down his inquiry on the grounds that he was not impartial, he set up a second inquiry and gave orders allowing Mr Williams in. This access was terminated at 2.30pm on Thursday, when officials at the centre told Mr Williams he had to leave. Mr Williams has consistently declined interview requests this week.

Police sources said that Mr Williams' efforts on Manus this week had been ''very helpful'' in their investigation, and said they were ''very sorry'' he had been deported. It is believed two PNG nationals are accused of striking the first and last, fatal blow to Mr Barati, while a group that included Australians are alleged to have attacked and kicked him when he fell after the initial hits to the head.

Mr Williams asked the court on Manus to begin contempt proceedings on its own motion before he was escorted by police from a separate jurisdiction and put on a plane for Port Moresby. Fairfax Media spent four days on Manus this week and spoke to many locals, who supported the economic spinoffs from the detention centre, but argued that the asylum seekers should have more human rights and more interaction with the local community.

A spokesman for G4S denied the company had failed to co-operate with PNG investigators. He said police had interviewed 10 expatriate G4S officers.