Shakeel Yusuf, a medical assistant at Srinagar's Sher-e-Kashmir Institute of Medical Sciences, was questioned along with the five others at the NIA's headquarters in south Delhi.
The NIA wanted to question Yusuf on receipt of funds sent by his father through a US-based international wire transfer company, an official said.
Yusuf was also "one of several Indian contacts" of hawala operator Aijaz Ahmed Bhat based in Saudi Arabia, and was said to have been in telephonic contact with him for receiving money transfer codes, said the NIA official.
The case, registered by the NIA in April 2011, relates to the transfer of money from Pakistan to Jammu and Kashmir through hawala channels via Delhi, which the agency believed was used in funding terror and secessionist activities.
The NIA has so far filed two charge sheets against six persons, including G.M. Bhat, a close aide of pro-Pakistan separatist Syed Ali Shah Geelani, Mohammed Siddiq Ganai, Ghulam Jeelani Liloo, and Farooq Ahmed Dagga. All four are in judicial custody.
Yusuf's father Mohammed Yusuf Shah -- better known as Syed Salahuddin -- was declared a Specially Designated Global Terrorist by the US Department of State in June last year.
Salahuddin also heads the United Jehad Council (UJC), a conglomerate of terrorist outfits operating in the Kashmir Valley.
The agency had last year arrested Salahuddin's another son Shahid in connection with the case. He is in judicial custody.