There's concerns over the cases of 25 people who were examined at the Sexual Assault Treatment Unit at Letterkenny General Hospital.
It's emerged a nurse working at the unit was not registered with the nursing board, An Bord Altranais.
The administrative blunder could potentially affect the outcome of the cases that the nurse was involved in.
Nurses are required to be registered with An Bord Altranais to practice but the nurse in question was removed from the register in 2009 for non payment of fees.
The HSE says it was notified at the time and that the information was made available within the organisation but that it didn't filter down to the Sexual Assault Treatment Unit in Letterkenny General Hospital.
It became aware of the issue in the last few weeks after a review at the Treatment Unit and says the nurse in question is no longer on duty.
Twenty-five people who were examined at the unit in the last 18 months were contacted yesterday by gardai and the HSE.
Gardaí say the matter has been brought to the attention of the Director of Public Prosecutions.
The HSE says it has appointed two independent experts to carry out a review to establish how a staff member could continue to practice without being registered.
Meanwhile, The Rape Crisis Network says what happened at Letterkenny General Hospital is "unacceptable".
Spokesperson Fiona Neary says these victims have already been through a traumatic experience: