NEW YORK (AFP) US interrogators grilled the Times Square bombing suspect for a fourth day Friday in a bid to resolve the puzzle of whether he has links to Pakistani jihadist groups, as he allegedly claims. Faisal Shahzad, 30, has still not appeared in court or been seen in public since his dramatic arrest late Monday on charges of planting a malfunctioning car bomb in New Yorks most crowded neighbourhood Saturday. A law enforcement source, who asked not to be identified, said Shahzad, who was hauled off a flight from New York to Dubai, was not scheduled to be arraigned Friday. Usually suspects appear before a magistrate judge within 48 hours of arrest, but they can waive that right, as Shahzad, a Pakistani-born US citizen, has apparently done. Officials will not comment on his detention or whether he has asked for a lawyer. The virtual disappearance of the alleged car bomber is adding to the mystery over how this formerly quiet, hard-working father of two from suburban Connecticut came to attempt mass murder in the heart of New York. Homeland Security Secretary Janet Napolitano told MSNBC television Friday that a pattern was emerging in the country of US citizens becoming radicalised. How that process works and what is the best way to interrupt it, to intervene (in) it, is something thats still a work in progress, she said. Shahzad allegedly has claimed that he was trained in bomb making by militants in the Waziristan region of Pakistan. Pakistani authorities say they are fully cooperating with the US probe, but that they have not yet found proof of Shahzads claim.