Intelligence gathered by foreign security services on returned Islamic State supporter Lisa Smith is a key part of evidence being put to the Dundalk woman in questioning.
Detectives are also quizzing the former Defence Forces member about comments and admissions she made in interviews in Syria.
The Garda investigation into Ms Smith, which has been ongoing since 2012, has involved detailed discussions with the Director of Public Prosecution regarding the legal basis for prosecuting the 38-year-old. Sources said the decision to arrest her suggests the DPP felt there were “strong enough” legal grounds to do so and, potentially, sufficient grounds to charge.
“The purpose of arresting her is to put the evidence to her and give her an opportunity to respond, to give her story, her alibi,” said one security source.
An “awful lot of work” has been going on with foreign agencies in gathering intelligence as to “where she was and what she was doing”, the source added .
This intelligence, from European and US agencies, provided gardaí with information on the whereabouts, movements, associations, and actions of Ms Smith, including in North Africa and Syria, where, from 2015, she lived in the so-called Islamic State.
She landed at Dublin Airport with her two-year-old daughter yesterday morning. She had been accompanied on a commercial flight from Turkey by two officials from the Department of Foreign Affairs and two members of the elite Army Ranger Wing.
Special Detective Unit officers arrested and detained her under Section 30 of the Offences Against the State Act, which allows for up to 72 hours’ detention.
In an interview with the Irish Examiner and RTÉ last October, the head of Garda Security and Intelligence, Assistant Commissioner Michael O’Sullivan, said they were conducting a criminal investigation into Ms Smith, on possible terrorist offences outside the state, under the Criminal Justice Terrorist Offences Act 2005.
He said: “We have avenues open to us, where we can get evidence and how we can get that evidence, and we are working closely with the DPP on how to put inform-ation we receive into evidence.”
He was confident of producing a “comprehensive file” for the DPP.
He recognised the 2005 act was “untested” in relation to prosecutions for terrorist offences committed outside Ireland.
One security source said that while a lot of the evidence may be circumstantial, trials have shown that juries can convict on circumstantial evidence.
“She may make admissions [in interviews] or she may have made admissions already in media interviews,” added the source.
Tusla said its priority was “the safety and wellbeing of the child”.
Prominent Muslim cleric Umar Al Qadri described Ms Smith’s return as a “critical situation”, with a jihadist attack on London Bridge and an increase here in Islamophobia and “right wing rhetoric in policing discourse”.
He said the “only reason she came back was that she had no other choice” and asked what guarantee there was that she now rejected the extremist Islamic State ideology.
Tánaiste Simon Coveney said the Government brought back Ms Smith and her daughter in light of the “dire conditions” the child faced in a camp, likening it to a “war zone”.