Intensifying its probe into the Artha Tatwa (AT) Group, the CBI on Thursday interrogated Chit Fund Depositors’ Forum joint convener Madhu Biswal. Sources said Biswal was managing the accounts of the AT Group before joining hands with the forum fighting for refund of money of duped investors. The CBI had summoned Biswal to depose before it over his involvement in the tainted chit fund firm. Earlier on Monday and Tuesday, CBI sleuths had grilled Youth Congress State president Rajat Choudhury for his alleged links with the AT Group.