The youths, identified as Akhlaq ur-Rehman, Mohammed Osama, Mohammed Azim Shah and Mehroz, were apprehended from Manglour town in Uttarakhand's Haridwar district by Delhi Police and produced in a court here yesterday which sent them to 15 days' police custody.
They are being interrogated jointly by sleuths of Delhi Police's Special Cell and a central intelligence agency, said a senior official.
The police have found evidence in connection with the youths contacting a former Indian Mujahideen operative who later went for training in Syria and is at present believed to be a key member of Ansar-ut Tawhid fi Bilad al-Hind (AuT), the official said.
"We are interrogating the youths and they have been confronted with certain facts but we are yet to ascertain the module or network which they are a part of," Special Commissioner of Police (Special Cell) Arvind Deep said.
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The four suspects were tracked down with inputs from a central intelligence agency. They had planned to carry out terror attacks at the Ardh Kumbh mela, trains heading towards Haridwar passing through Roorkie and some strategic locations in Delhi, police said.
The accused were under the radar of security agencies after their suspicious online activities were detected about a few months ago, officials said.
The youths used to regularly visit ISIS-supported websites, Facebook group and were in regular touch with some supporters of the terror group through WhatsApp.
The suspects were in touch with motivators supposedly located in ISIS territory in Iraq-Syria, resulting in their coming under the surveillance of security agencies, officials added.