"I will appoint a Special Presidential Commission of Inquiry this week to investigate the Central Bank bond issue and recommend actions," President Sirisena tweeted.
The gazette notice is to be issued this week with the panel being given three months to conclude the report and specify legal action if any.
The three-member Presidential commission of Inquiry will have a judge from the Supreme Court, High Court and an auditor, the statement said.
When Mahendran's tenure ended in June last year, Sirisena denied him an extension and overlooked Prime Minister Ranil Wickremesinghe alternative candidate.
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The firm was accused of benefitting from insider information in the two bond issues in February 2015 and March 2016.
It was alleged that auctions were rigged where larger than announced volumes were sold, at low prices. The state has suffered large losses from the 30 year Bonds.
Parliament is scheduled to debate the report by the COPE on the alleged bond scam this week.