Kolkata: CBI on Thursday said that interrogating Trinamool Congress all-India General Secretary Mukul Roy was critical for framing of chargesheets against the key accused who were now under arrest.

"We do not want to delay the case inordinately since the probe is in an advanced stage. So we want to interrogate Roy at the earliest so that chargesheets can be framed against the key accused in Saradha Realty case on time," a CBI source told PTI.

Roy, a former railway minister, was served summons on Monday to appear before the investigating agency by this week but had sought for a 15-day extension for appearance.

He would be interrogated for his role in alleged criminal conspiracy for diversion of funds in the Saradha ponzi scam. The source said that after much negotiations with Roy, he had been asked to appear before the CBI by January 21, 2015.

"We also told him that we cannot delay," the source said.

He said that quizzing Roy would help in framing chargesheets against minister Madan Mitra, Trinamool Congress Rajya Sabha MP Srinjoy Bose and others in the Saradha Realty case.

"If we are not able to file the charge sheet within the stipulated time after their arrests, then these persons will get default bail and we will be blamed unnecessarily," the

source said. "Somethings are time bound," he added.