The Department of State Services (DSS) recovered incriminating materials when it searched the residence of Mr Tukur Mamu.
Mamu is the negotiator between the Nigerian government and terrorists who abducted passengers from the Abuja-Kaduna train service of March 28.
DSS spokesman, Dr Peter Afunanya said appropriate security agencies executed the search warrant.
He stated in Abuja on Thursday that incriminating materials recovered included military accoutrements, large amounts of different currencies in different denominations and financial transaction instruments.
Afunanya assured that Mamu would have his day in court to defend himself.
The DSS on Wednesday confirmed the detention of Mamu in its facility.
Mamu was intercepted by Nigeria’s foreign partners in Cairo on Tuesday as he was jetting to Saudi Arabia.