Agarwal is suspected to have acted as a middleman with SEBI and other regulatory bodies and saved Sen from their action, CBI sources said.
Sen is believed to have told CBI that Agarwal had taken a substantial fee for helping him keep SEBI and other regulatory bodies from acting against him, they said.
CBI sources said it was felt during the probe that Agarwal's custody was important for further investigation and added that he was arrested late in the evening today.
Besides, there are 44 FIRs in Odisha against ponzi companies.
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Supreme Court had handed over the Saradha chit fund scam probe to CBI and asked the state governments to provide all logistical help to the agency.
CBI has formed a Special Investigating Team (SIT) headed by Joint Director Rajiv Singh to also probe the role of the Securities Exchange Board of India (SEBI) and RBI.
"The investigation conducted so far puts a question mark on the role of regulatory authorities like SEBI, Registrar of Companies and officials of RBI within whose respective jurisdictions and areas of operation the scam not only took birth but flourished unhindered," an apex court bench headed by Justice TS Thakur had said in its order.