DSP, Jalpaiguri Probhat Chakraborty told PTI that the NIA team collected samples from the blast site.
No one had been arrested or detained by the police till now, he said.
The DSP, however, refused to comment when his attention was drawn to an SMS circulated purportedly by the Kamtapur Liberation Organisation (KLO), suspected to have carried out the blast, denying its involvement.
"We won't say anything on this matter now. All angles to the blast are being investigated," Chakraborty said.
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Asked if there was a prior intelligence alert about the blast, Anuj Sharma said, "As the KLO was observing its martyrs' day yesterday, we had issued a general alert requiring us to conduct checks at various check points."
On whether it was an intelligence or security failure, Sharma replied, "I won't say anything right now. The probe is on. The CID and IB are looking into the matter."
The needle of the blast pointed to the KLO, an organisation formed in 1993 to fight for a separate Kamtapuri state comprising six districts of north Bengal and some areas of lower Assam.