Mr Zeliang has been asked to appear at the NIA's headquarters in New Delhi tomorrow. However, he has written back to them saying that since the state assembly session would start on Tuesday, he would not be able to go to NIA's office in the national capital.
Fourteen government departments in Nagaland are under the NIA's scanner, people familiar with the matter said. In October 2017, the NIA arrested four government officers in an alleged terror funding case.
In January last year, the NIA conducted searches at government departments in Nagaland and seized receipts that showed payments to the tune of Rs 2 crore. The probe agency believes that government money could have made its way to the coffers of terrorists with help from officials in key posts.
Mr Zeliang's Naga People's Front said it was a case of political vendetta.
"A devious game is being played against former chief minister and Naga People's Front leader TR Zeliang by the National Investigation Agency which is tasked with looking into terror activities in the country, but now appears to be used to grind the proverbial political axe," the NPF said in a statement.
"With the announcement of state assembly elections 2018 in Nagaland, came a summon from the NIA for three staffers of the then chief minister's office. The three officers... complied and appeared before the NIA officer concerned at the Guwahati NIA branch office," the NPF said in the statement.
"However, just a day before the declaration of the assembly elections result, one of the officers from CMO was again summoned to appear before the NIA investigating officer... he was repeatedly interrogated for five days from morning till late evening, until the day TR Zeliang resigned from the office of chief minister," the NPF said in the statement.
In 2016, S Khetoshe Sumi, a senior officer of the armed group Nationalist Socialist Council of Nagalim (Khaplang) or NSCN(K), was detained by Assam Rifles troopers in Dimapur. His interrogation led to the NIA getting information of possible terror funding in Nagaland.
In November 2017, massive protests rocked Kohima after seven government officers were reportedly arrested for funding armed groups, called Naga National Political Groups, through 'taxation'.