Chidambaram, however, declined to elaborate further, saying it "can't be disclosed to you".
As many as 68 people were killed when RDX-laden explosives were triggered in the Samjhauta train, the oldest rail link between India and Pakistan, in Haryana on the night of February 17, 2008.
The NIA had taken the custody of Abhinav Bharat member Swami Assemanand in this connection from a Punchkula court and were finding links as to who had supplied the RDX.
Asked for comment to the demand made by Congress general secretary Digvijay Singh that all terror-related cases should be probed by the NIA, the home minister said that the Central probe agency was already investigating a number of terror cases.
"Not all cases where the UAPA (Unlawful Activities [Prevention] Act) provisions were invoked can be handled by NIA. NIA has the constraint of manpower, time. But all major cases of terror are being investigated by NIA and in future too all major terror cases will be handled by NIA," he said.