Karnataka police have been coordinating with their Tamil Nadu counterparts on several terror-related cases, including the one involving the April, 2013, low-intensity blast near the BJP office at Malleswaram in Bangalore.
All the accused in that case, against whom a charge-sheet was filed last year, are from Tamil Nadu. Among them were Panna Ismail and Bilal Malik, who were arrested in October, 2013, from Puttur in Andhra Pradesh.
CB-CID officials have already completed a round of probe in Bangalore and questioned two suspects there.
They have also stated that the twin blasts on the train was similar to the Patna explosions at the venue where BJP leader Narendra Modi addressed an election meeting.
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Officials have indicated that "every angle will be probed".
One woman was killed and 14 others injured, two of them critically, when two low-intensity bombs exploded in two coaches of the Bangalore-Guwahati train at the Central Railway station here on May 1.