Hamsa, 52, was the main man for indoctrinating and recruiting for ISIS, say Kerala police sources. More arrests are expected in the coming weeks.
During his interrogation, Hamsa reportedly dared the police to call in a religious expert to prove his assertion that the "ISIS is true Islam".
The police say Hamsa had operated from Gulf countries since 1998. "He worked at a Bahrain-based religious centre Al Ansar, which we suspect to be a training centre for ISIS sympathisers. Many Indians who are suspected to have joined ISIS, took this route to Syria," police sources said.
42-year-old Manaf Rahman, arrested along with Hamsa, was influenced by him and was supposed to leave for Syria six months back, say sources.
He was turned back at the Mangaluru airport based on suspicion. Hamsa was supposed to leave the country after Manaf, but he too, had to stay back.
"We have formed a special team in Kannur to track terror-related modules including ISIS networks. These arrested have been questioned many times before. We have been in coordination with NIA (National Investigation Agency) as well as other state police forces to track the activities and movement of suspects. There are more under our scanner," Kannur Police Chief G Siva Vikram told NDTV.
Kannur police said they were headed to Syria to join the terror organisation. When their online activities were tracked, it was found that they had been active on websites with links to ISIS, police officials said.
The three have been identified by the police as Mithilaj KC, 26, Abdul Razak KV, 24, and Rasheed MV, 23.
An estimated 100 Indians have left to fight for ISIS in Syria, Iraq and elsewhere, many of them from Kerala amid concerns that an extreme form of Islam was taking root in the state.
The National Investigation Agency has been probing cases of disappearance of at least 21 people are suspected to have joined the ISIS.