The Indonesians who recently claimed they had escaped from a camp run by the Islamic State in Iraq and Syria (ISIS) in Raqqa were loyalists of the militant group, according to a police source.
The group of 18, said to be a family from Jakarta, were repatriated from Syria and arrived at Jakarta's Soekarno-Hatta International Airport on a Qatar Airways flight on Saturday.
They were immediately detained and are now being questioned by police investigators from Indonesia's elite counter-terrorism unit known as Densus 88.
"They had pledged allegiance to ISIS when they were there and had been indoctrinated, so we want to conduct a thorough investigation," a Densus 88 commander told The Straits Times yesterday.
The officer cannot be named due to the nature of his work.
"They had claimed to have been duped, saying they are innocent but we don't want to just believe them.
"So we want to know in detail, how they were 'offered' to go to Syria, who contacted them first, and whether any of them were part of a terrorist network in Indonesia," he added.
Earlier reports had said the family comprised 17 Indonesians.
But a police operations report seen by The Straits Times indicated that the Qatar Airways flight, which landed in Jakarta at 3.30pm local time on Saturday, was carrying 18 Indonesians.
They comprise three men; six women; five teenagers, three of whom are girls; three children and an infant.
The Straits Times understands that the group will not be released after questioning but will have to undergo a deradicalisation programme run by the National Counter-terrorism Agency.