The father of the first woman detained under the Internal Security Act (ISA) regrets not reporting his daughter to the authorities.

Mr Syaikh Abdu Manaf Al Ansari told Berita Harian last night that he and his wife had questioned Syaikhah Izzah Zahrah Al Ansari in late 2014, after noticing she had started dressing in black and wearing the niqab, a facial veil which reveals only the eyes. She was also using the Islamic State in Iraq and Syria (ISIS) flag as her WhatsApp display picture, he said.

"I asked if she was an ISIS member. She denied it but said she thought ISIS was fighting for Islam," said Mr Manaf, 49.

"I told her all well-known Islamic scholars reject ISIS. I asked, is it halal in Islam to kill innocent people, children and women? I told her to show me evidence from the Quran and sunnah (prophetic teachings) that it is halal. She just kept silent."

Izzah was detained earlier this month under the ISA for radicalism.

Mr Manaf and his wife, both Quranic teachers, gave Izzah - the second of five children - religious advice, and decided not to inform the authorities after she stopped wearing the niqab and started listening to music and watching films again.

"We thought she was okay. But we did not realise she had become more radical. She was smart at hiding herself," he said.

After Izzah was arrested, he found documents in her room with information about moving to Syria, including how to get tickets.

He felt "disgust and anger" and, in a bid to "protect her", threw the materials away. He told the authorities about them only during the investigation, and was given a warning.

"This should not have happened. I really regret it," he said.

He advised the public to contact the authorities or the Religious Rehabilitation Group (RRG), which helps to counter radical ideology, if they notice suspicious changes in their loved ones. "To other parents, this comes from the bottom of my heart. If you see any sudden changes, et cetera, contact the authorities. If not (the authorities), contact RRG," he said.