A council has launched an inquiry into how failures in its schools criminal checking system were obtained by BBC Wales.
Some people working with children in Powys had not been checked
Powys Council said it had uncovered "weaknesses" in its Criminal Records Bureau procedures after inquiries by the BBC Wales News website.
The council later met in closed session, and has confirmed it is investigating the leak.
A review found that 44 school workers were not cleared to work with children.
The council said the majority were peripatetic teachers, and that steps had been taken to ensure that no-one had unsupervised contact with children while the necessary checks were carried out.
A council spokesman said: "We can confirm that the council is holding an inquiry into how information was released to the media.
"We can't comment further until the inquiry is complete."
Conservative Mid and West Wales AM Glyn Davies criticised the inquiry.
Powys Council met in closed session at its headquarters
"The council is spending too much time chasing the whistleblower when it ought to be concentrating on its criminal record check failures," said Mr Davies.
"It seems to me that this person put something into the public domain that should have been there anyway."
Last week, Powys Council said the majority of workers not cleared to work with children were peripatetic teachers.
It said 39 employed before the Criminal Records Bureau (CRB) came into force in 2002 had been overlooked, and a further five had not been checked since then.
Last Thursday, Education Minister Jane Davidson said she was shocked, while the National Association of Schoolmasters Union of Women Teachers (NASUWT) said it was astounded.
The following day, the Welsh Local Government Association (WLGA) said it would contact all local councils in Wales to ensure that everyone working with children had been checked by the CRB.
It was initially thought that up to 80 people had slipped through the net unchecked in Powys, and the council brought in extra staff "to make sure the review is carried as quickly as possible".
The CRB runs checks for organisations which are required by law to vet people working in positions of trust, often with children.