Police have been examining Mr Howell's patient records
Police have been tracing patients of a dentist charged with a double murder and a string of indecent assaults.
Colin Howell, 50, from Sea Road, Castlerock, is accused of the double murder of his wife Lesley and RUC officer Trevor Buchanan in 1991.
He was charged with indecent assault and administering an overpowering drug with intent to commit indecent assault.
At least 200 letters will be sent out to patients by police seeking help with their investigation.
Detectives have been checking Mr Howell's records to find details of his appointment lists.
A police spokesman said: "This named individual is under investigation. He has been charged with a number of serious offences. It would be inappropriate for police to comment further at this time due to the ongoing investigation and future court proceedings."
Mr Howell has been in custody charged with the murders of his wife Lesley Howell and Trevor Buchanan, whose bodies were found in a fume-filled car in Castlerock in May 1991.
At the time, a coroner's court ruled that both had committed suicide.
Mr Buchanan's former wife Hazel Stewart is also charged with the murders.