The National Investigation Agency (NIA) on Thursday took custody of three doctors and a preacher who were arrested in connection with the November 10 car blast outside the Red Fort in which 15 people were killed, officials said.

Muzammil Ganaie, Adeel Rather and Shaheena Saeed as well as Maulvi Irfan Ahmed Wagay had been arrested by the Jammu and Kashmir police following the blast.

"They had all played a key role in the terror attack that killed several innocent persons and left many others injured," an NIA spokesperson said.

With their custody shifting to the NIA, which formally took over the case on November 11, the number of people booked by the federal agency has gone up to six.

The NIA has already arrested two people -- Amir Rashid Ali and Jasir Bilal Wani alias Danish.

Dr Umar-un-Nabi, who was driving the explosives-laden i20 that detonated, had allegedly bought the car in Ali’s name. Wani was arrested after it emerged that Umar had been trying to convince him to become a suicide bomber. He was not persuaded but is alleged to have agreed to participate as an overground worker for the banned terror outfit Jaish-e-Mohammed.

Those arrested are alleged to be at the centre of a 'white collar' terror module busted by Jammu and Kashmir police along with their counterparts in Uttar Pradesh and Haryana. Investigations led to the Al Falah university in Faridabad where 2,900 kg of explosives was recovered.

It all started on the intervening night of October 18-19, when posters of the banned JeM surfaced on walls just outside Srinagar city. The posters warned of attacks on police and security forces in the Valley.

Srinagar police decided to treat the matter as a serious issue and not just as a one-off incident and Senior Superintendent of Police (Srinagar) G V Sundeep Chakravarthy formed several teams to get into the depth of the case.

Three people -- Arif Nisar Dar alias Sahil, Yasir-ul-Ashraf and Maqsood Ahmad Dar alias Shahid -- were arrested after CCTV footage showed them pasting the posters.

During interrogation, they named former paramedic turned preacher Maulvi Irfan Ahmed from Shopian as the one who had supplied the posters. He was arrested.

This was the thread that led to the unravelling of the plot. It was his interrogation that finally led investigators to Al Falah university and the group of Kashmiri doctors.

The first person to be arrested was Ganaie from Faridabad. Then Saeed too from the town. Later, Adeel Rather was picked up from Saharanpur in Uttar Pradesh.