A senior Garda officer is still being questioned tonight as part of an investigation into the unlawful disclosure of information to the media.
His period of detention was extended by a further 12 hours.
Investigators have visited a number of locations, including garda offices where the man worked, and the offices of a national newspaper.
They have interviewed and taken statements from a number of gardaí who worked with the man and also sought but did not succeed in interviewing a journalist
The investigation began last year following a report by the former Children's Ombudsman Emily Logan into the removal by gardaí of two children from their families in Athlone and Dublin in October 2013.
The report found that while the Health Service Executive could not be ruled out, the release of details in relation to one of the children probably came from someone within the Garda and may have been an offence.
The officer was arrested this morning after he was suspected of breaching the Garda Síochána Act. His period of detention was extended this afternoon.
RTÉ News has learned that he was authorised to speak to the media at the time the children were removed from their families.
He is the most senior garda officer ever to have been arrested as part of a criminal investigation and the Government has been informed.
Garda Commissioner Nóirín O’Sullivan has suspended him from duty.
The Garda Press Office would neither confirm nor deny the officer's arrest.
In a statement, it said that the investigation into the disclosure of confidential and sensitive information was ongoing.
The garda was arrested after he arrived at a Co Dublin garda station by appointment at 7.30am this morning.
He is suspected of unlawfully disclosing information to the media likely to have a harmful effect.
Commissioner O'Sullivan established the inquiry last July following the publication of a report by Ms Logan into the use of legislation by gardaí in Tallaght and Athlone to remove two children from their families and place them in care.
The children were subsequently returned to their families.
However, Ms Logan's report found that while the HSE could not be categorically ruled out as the source, the publication in an article of details in relation to one of the children "on the balance of probabilities" came from someone within an Garda Síochána and may have been an offence.
The investigation is being led by a Chief Superintendent and a Detective Superintendent based in Dublin city centre.
The office of the garda in custody has been searched and his phone and computer records examined.
This evening the National Union of Journalists has said it is concerned about what appears to be a number of unusual aspects in relation to the arrest of a senior member of the gardaí for the alleged unlawful disclosure of information.
National Secretary Seamus Dooley said it is vital that journalists are allowed to do their work in the public interest and that further barriers to the release of information are not erected.