POLICE investigating widespread assaults on Vietnamese detainees during the controversial Whitehead detention centre raid say there is insufficient evidence to justify criminal charges against officers. Senior sources said yesterday it would be ''highly surprising'' if any basis for prosecution could be found. This is despite an official government report, completed in June by two Justices of the Peace, that concluded that an unknown number of assaults took place on April 7 in the pre-dawn raid by police and Correctional Services Department (CSD) officers. More than 500 canisters of tear-gas were fired and more than 200 Vietnamese injured in the raid. The operation was aimed at transferring detainees and segregating a smaller group for possible deportation. ''It would be highly surprising to us if they found anything,'' one investigator said. ''But we all feel this might have developed into something of a political case, a political issue, and if they want to charge someone, they will go ahead and do it anyway. ''Sometimes, the Legal Department will take the view that very contentious issues should be aired in court.'' The final police report was passed to the department on Monday. A spokesman confirmed the report was being studied, but declined to say when a decision would be made. Unlike some criminal inquiries, the police report specifically evades making recommendations on the prospect of charges; an official decision taken because of the sensitive nature of the matter. However, police claim there is little corroborative material to support any specific prosecutions. In the past 14 weeks, 250 Vietnamese refugees, police and CSD officers have been interviewed in relation to al legations of assault and theft. More than 18 hours of video footage - filmed during the raid by members of the 1,200-strong raid contingent - was also studied. Large teams of translators were required to compile interviews and more than 50 identification parades were conducted. The strongest supportive evidence is believed to have come from photographs taken of bruisings suffered by inmates during the ordeal. But in some cases there was a large time lapse between the assault and the photo sessions and this has apparently complicated the process of gathering evidence. Police also claimed some victims frequently changed their statements. Despite the government report's findings of excessive use of force, numerous assaults and ineptitude in releasing information on the raid, no CSD or police officers have been suspended from duty. Possible disciplinary action will be reserved until after a decision is made on whether to lay criminal charges. The Director of Operations, Senior Assistant Commissioner Toby Emmet, said last night police had completed a review to help prepare for future operations aimed at repatriating the refugees. ''Having reviewed our role, we were quite happy with our role,'' he said. Mr Emmet said that police - in the wake of criticism over wrong figures being released by officials - had introduced procedures to more quickly estimate the number of rounds of tear-gas fired. He said police also had no objection to independent observers taking part in future operations. Last night, a spokesman for Refugee Concern, Pam Baker, said civil cases were being planned for claims in relation to assault and theft. The Legal Aid Department would be asked to take on the cases of those injured when they fell off a roof, she said. ''I feel sure there will be some criminal charges laid against officers who have been identified,'' Ms Baker said. ''This will be of assistance in the civil claims.''