The NIA, which has taken over the case from Manipur police, conducted a thorough search of the area, five days after the attack, the worst on the army in recent times, official sources said.
They said the IED was defused by explosives experts from the army.
Naga insurgent outfit NSCN (Khaplang) had in a release dated the day the attack took place, claimed responsibility for the ambush which left 18 army personnel dead and 10 others injured.
NIA has taken over the case from Manipur police and registered a case in the agency's Guwahati branch under various sections of the Indian Penal Code, Arms Act and Unlawful Activities (Prevention) Act.
According to the intelligence reports, ULFA's fugitive chief Paresh Baruah, who is alleged to be opertaing at the behest of China's PLA, is believed to have convinced NSCN-K leader S S Khaplang to snap the ceasefire agreement with Indian government in March last year.