Mohammed Majid, one of the four arrested on Friday, in connection with the 7/11 Mumbai train blasts, was an active member of the 'sleeper cell' of Lashkar-e-Taiba and a mastermind behind the conspiracy, police sources said in Kolkata on Saturday.
A joint team of Mumbai police's Anti-Terrorism Squad (ATS) and the Kolkata police special branch raided Majid's hideout in East Kolkata's Rajabazar area on Friday and flew him to the western metropolis for further interrogation, sources said.
Majid was hiding in the area after escaping from Aurangabad in the wake of massive arms haul in May last, they said.
He was suspected to be involved in arms smuggling, they said adding he was believed to have provided shelter to some other suspected terrorists in a house in the city at that time.
Majid was part of a group of LeT operatives who used to take order from their western India commander SK Faisal.
The arrest of Majid, the LeT operative, in Kolkata and three others from Thane came in the wake of disclosure by Faisal, arrested earlier, during multiple narco-analysis test, sources said.
City police sleuths said there were also investigating whether more persons were linked with Majid in Kolkata. "We are in close touch with the ATS in this regard," they said.
A series of explosions occurred in local trains in Mumbai in the peak hours of July 11, killing about 200 persons and injuring over 600.