Some of the Indian Mujahideen members who carried out the Mumbai serial blasts may have fled to Kolkata, believe investigators.
The police are now closely examining passengers who have traveled to Maharashtra from Kolkata in the last ten days.
"We are still looking for leads. The Kolkata police have been informed about the latest development. The Kolkata special task force, which is assisting in this probe, has indicated that some members of the IM had gone off their radar a couple of days ago," said an official involved with the investigations.
Many cadres of the IM have used Kolkata as a hide-out due its proximity with Bangladesh. Arrested IM members, during their interrogation, have confessed to using this route.
T Nasir, the prime accused in the Bangalore blasts, had also sought refuge in Bangladesh before he was nabbed by the police on the border.
The IM may even have used migrant labour from border areas to carry out their operation as was done during the Varanasi blasts.
Some of these illegal migrants, who have no records, execute the terrorists' tasks for money and they are virtually impossible to catch.
Investigators are carrying out raids in Gujarat, Jharkhand, Madhya Pradesh and also Karnataka in search of key IM operatives.
"We are conducting raids and also studying the various interrogation reports of IM members. We are examining every possible angle," said an official of the National Investigation Agency.