New Delhi:The National Investigation Agency (NIA) on Thursday took custody of three doctors and a preacher arrested in connection with the November 10 car blast outside the Red Fort, which killed 15 people, officials said. The three accused had been arrested by the Jammu and Kashmir police following the blast. According to the PTI report,anNIAspokesperson said,"They had all played a key role in the terror attack that killed several innocent persons and left many others injured". Also Read:Delhi doctors with foreign degrees under investigation in terror probe With their custody shifting to theNIA, 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. The accused doctor, who was driving the explosives-laden i20 that detonated, had allegedly bought the car. Another accused was arrested after it emerged that he had been approached to become a suicide bomber. Although he refused, he is alleged to have agreed to act as an overground operative for the banned terror outfit Jaish-e-Mohammed (JeM). 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 theAl Falah Universityin Faridabad, where 2,900 kg of explosives were recovered. It all started on the intervening night of October 18-19, when posters of the banned JeM appeared 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 depths of the case. Three people were arrested after CCTV footage showed them pasting the posters. During interrogation, they named a former paramedic turned preacher 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 toAl Falah Universityand the group of Kashmiri doctors. The first arrest was made in Faridabad, followed by another in the same town. A third individual was later picked up from Saharanpur, Uttar Pradesh. Also Read:Delhi Blast Probe: 200 doctors, staff of Al Falah University under investigation