The Belgian financial regulator has sent letters to 640 senior executives connected to Fortis as part of a wide-ranging investigation into insider trading at the financial group.
Authorities are investigating transactions made around the time of the bancassurance group's dismantlement in October 2008.
Most of the letters were sent to senior managers across the remaining parts of Fortis, but not to those that sat on the group's executive committee, according to L'Echo, the Belgian financial daily.
Shares in Fortis tumbled shortly after the collapse of Lehman Brothers in September 2008, amid concerns of the solvency of its banking operations. The group was hastily broken up and part nationalised shortly afterwards, with BNP Paribas taking over the banking operation.
The financial regulator is currently investigating a raft of insider trading claims relating to Fortis. It stressed that the recipients of the letters were not suspected of wrong-doing.
The fall-out of the collapse of Fortis caused the collapse of the Belgian government at the time, over claims of interference in the judiciary.
Fortis Holding and BNP Paribas declined to comment.