Security agencies probing the Samjhauta Express blasts on Tuesday night said the incident was the handiwork of Pakistan-based militant groups.
Highly-placed sources in Union Home Ministry said they had "vital clues," which included a telephonic conversation between a caller from Delhi to Pakistan-occupied Kashmir.
The sources declined to give out the identity of the militant groups behind the blasts, but hinted at sleeper cells of Lashkar-e-Tayiba and Jaish-e-Mohammed.
These militant groups, the sources said, have adopted a new modus operandi of detonating low-intensity blasts with explosive materials comprising readily available combustibles.
The matter would be taken up with Pakistan authorities who would also be apprised of the investigation into the blasts, the sources said, but ruled out it would be part of the agenda of the meeting between External Affairs Minister Pranab Mukherjee and his Pakistani counterpart Khurshid Mehmood Kasuri on Wednesday.