TWO Vietnamese boat people are being held in solitary confinement while police investigate the death of their former cell-mate in Lai Chi Kok detention centre. An autopsy yesterday revealed that Tran Diem Dung, 24, died from serious internal injuries caused by heavy blows, although police last night were still treating the case as a ''person collapsed''. Correctional Service Department (CSD) staff went to the cell holding the three men on Saturday night after screams were heard and found Tran complaining of chest pains before he collapsed. He was taken to Princess Margaret Hospital where he was certifieddead on arrival. Tran, a former inmate of Whitehead detention centre, had been remanded in custody since April on a charge of wounding and had been in good health. The other two men had also been charged with wounding after an incident at Whitehead in July. It was not known last night how long the three men had been in the same cell. A CSD spokesman confirmed the death and said the matter was in police hands. He said there were no plans for a review of staffing or security procedures. at Lai Chi Kok, which is used as a remand centre. A police spokesman for the Kowloon West crime unit said the investigation was ongoing, but declined further comment.