Police reports of the threatening letters were received between 27 Apr and 4 May this year.
The letters related to the closure of the Sungei Road flea market, and were sent under the name of the flea market association’s chairman, Mr Koh Eng Koon.
Among the ministers to receive the notes were PM Lee Hsien Loong and DPM Tharman Shanmugaratnam.
Investigations are ongoing.
If convicted of criminal intimidation, the two can be jailed for up to 2 years, or fined, or both.