Operation Pallial is being conducted by the National Crime Agency, at the request of North Wales Police, to ensure its independence.
North Wales Chief Constable Mark Polin said he had asked the NCA to continue the work to phase two.
I took the decision to ask the NCA to investigate these allegations, conscious that some victims of historic abuse may not have the necessary level of confidence in North Wales Police to report matters directly to us.
Pallial has now secured accounts from almost all victims who are willing to support an investigation and it makes absolute sense for the officers and staff involved to be at the core of phase two and to move matters forward as quickly as possible.
Phase two will involve further investigations, he said, in liaison with the Crown Prosecution Service.
More top news
Patchy rain affects northern areas of the UK in the next 24 hours. Elsewhere, dry but rather cloudy.
Marjorie and Micheal Cawdery, both 83, were found dead in their house in Portadown, Northern Ireland on Friday.
Ann Myatt, who is contesting Jo Cox's former seat, told a hustings: "We've not yet shot anybody so that's wonderful."