Jan. 24 :    Jan. 24:  Aftab Ansari's mother Mohsina and sister Ruksana are being interrogated by police in Uttar Pradesh in search of leads to the attack in front of the American Center in Calcutta on Tuesday. Ansari, also known as Farhan Malik, had claimed responsibility for the strike, in which motorcycle-borne gunmen fired on a group of policemen, killing four. Delhi today announced it was seeking the don's extradition from the UAE. The questioning of Ansari's family members is part of a growing investigation that now spans at least three states - Bengal, Uttar Pradesh and Bihar, where the attackers may be hiding. Returning from his trip to Calcutta, the special secretary in the Union home ministry, Ashok Bhandari, has reported that the killers - either in Purnea or Kishenganj in Bihar - may try to slip into Nepal. Investigations have revealed that the blueprint for the attack could have been prepared by an ISI agent operating from Bihar, where the Pakistani agency is suspected to have set up a well-organised network. Purnea is a well-known centre of illegal gun-making and gun-running. The Bihar chief secretary and police chief are now in Delhi and an IB team from the capital has gone to the state. 'We have fresh inputs that the killers fled Calcutta after the massacre and have taken shelter along the narrow Kishenganj-Siliguri corridor,' an intelligence official said. The 28-km corridor in the north of Bengal, also known as the Chicken's Neck, is hemmed in by Nepal to the west and Bangladesh to the east. 'Since the outfit's Bangladesh links have been established and possible exit routes into that country sealed, the group will try to slip into Nepal. The killers are being sheltered by the fundamentalist organisations in the region,' the official said. In Ansari's hometown, Varanasi, the police have picked up a number of people and are interrogating his friends and neighbours in the Lallapura area. 'But the key is Mohsina and Ruksana,' a home department official in Lucknow said. The police are 'sure' the family knows Ansari's whereabouts, he added. The police claim to have 'good leads' that could crack the Calcutta conspiracy, but are not ready yet to disclose details for fear of jeopardising a parallel inquiry into what they describe as Ansari's plot to disrupt the polls in Uttar Pradesh next month. They have charted Ansari's transformation from a student at Benaras Hindu University, where he studied law but did not complete his degree, to a member of the Dawood Ibrahim gang with access to some of the world's most dreaded terrorists like Omar Sheikh, who was freed from Tihar jail with Masood Azhar to secure the release of passengers of the Indian Airlines plane hijacked to Kandahar. He is supposed to have a 'nikat ka rishta' (close relation) with Sheikh. Neighbours note the change in the family's lifestyle - describing it as 'drastic' - from 1999, when Ansari quit the university, till today. Although the police claim he is still sending money to his mother, sister Ruksana denies the charge. Mohsina would only say he was always a 'good son'. Investigators say Ansari has been in touch with his family as he pursued the life of a terrorist-criminal. His first big criminal act was thus far known to be the kidnapping of a diamond merchant in Gujarat, but the police here also cite the abduction of Anand Prakash Agarwal, a coal merchant in Varanasi, in 1999. They say Ansari collected a ransom of Rs 2.75 crore in that case.