The person identified as Mohammed Ayub had figured during the interrogation of Wasim Akram, a medical student who is being touted as the main accused in the case by the probe agency, official sources said.
Ayub is alleged to have purchased a SIM card for a known Hizbul Mujahideen commander - Jehangir -- one of the terrorists whom the NIA was looking for in connection with the conspiracy.
It is not clear whether Ayub would be placed under arrest and brought to the national capital or would be questioned in Kishtwar itself and handed over to local police, the sources said.
Wasim's uncle and cousin were also summoned to Delhi and questioned after their numbers figured in the phone book of the accused.
At least 15 people were killed in the blast outside the Delhi High Court on September seven last year.
Meanwhile, a massive search operation is underway in the higher reaches of Kishtwar for Junaid Akram Malik, wanted in connection with the case.
NIA has already announced a cash reward of Rs 10 lakh each for information leading to the arrest of Junaid and his two accomplices -- Shakir Hussain alias Chota Shakeel and Amir Ali Khan alias Amir.
Junaid's brother Wasim Akram is at present in judicial custody.