The Police Ombudsman is starting a criminal investigation into five PSNI officers for alleged misconduct over the Cookstown disco crush tragedy.
Four of the officers attended an incident at the Greenvale Hotel in County Tyrone where three teenagers died on the night of 17 March.
The other officer was involved in call handling that night.
The PSNI had asked the ombudsman to look into the actions of the first officers arriving at the hotel.
The ombudsman's office confirmed it was considering whether the five officers committed the offence of misconduct in public office.
Morgan Barnard, 17, Lauren Bullock, 17, and Conor Currie, 16, died as hundreds of young people queued to get into an event on St Patrick's night.
The PSNI said at the time that officers who responded to a 999 call "withdrew to await further police support".
The then Chief Constable Sir George Hamilton has since described the officers' actions as "brave" but later apologised for doing so.
Morgan Barnard's parents have said serious questions must be asked of the police.
The inquiry "does not come as a surprise", according to the Barnard family's solicitor Darragh Mackin.
The ombudsman's decision to investigate "exonerates the family's efforts to ensure that no stone has been left unturned in the pursuance of truth", he said.
No officers suspended
The PSNI said it would "cooperate fully throughout" the ombudsman's investigation.
"We have full confidence in the office of the Police Ombudsman to complete a thorough and independent investigation," said Deputy Chief Constable Stephen Martin.
"Until this is complete it would be inappropriate to comment further.
"Our thoughts remain with the families of Morgan Barnard, Connor Currie and Lauren Bullock who tragically died at the event and the police investigation into the circumstances surrounding their deaths continues."
It is understood none of the officers under investigation has been suspended but that position is being kept under review.