India, April 7 -- In a cramped rented home in Kurla's Abdulla Compound, a father sits with his head in his hands, struggling to make sense of how his 18-year-old son's life unravelled in a matter of days.

Mohammad Hamad Jalaluddin Mehmood Ali Siddique, barely out of school and having just turned 18, is now in the custody of the Delhi Special Cell, arrested with the help of the Maharashtra Anti-Terrorism Squad (ATS) over alleged links to an Islamic State (IS)-inspired module that investigators claim was planning attacks using improvised explosive devices.

For his father, Hamad Siddique, who works in the real-estate, the allegations feel unreal and unbearably heavy. "My son is innocent. He was in Higher Secondary Board (XIIth) and just co...