P Chidambaram, senior Congress leader and former union finance minister, could be called in by the Central Bureau of Investigation for questioning, sources said today, even as his son Karti Chidambaram was ordered by a Delhi court to spend three more days in custody of the agency. The CBI had asked the court to extend the custody of Karti Chidambaram by nine days, saying it has "concrete credible evidence collected recently" against him in a corruption case.