Each family living opposite the Judges' Court at Palashpole received a letter each on Tuesday, asking them to leave.
They, however, did not file any complaint with the police until late on Tuesday night.
Satkhira Sadar Police Station OC Enamul Haq said they were investigating.
The Jamaat-e-Islami had unleashed massive violence in the district after their leaders were awarded death or life sentences by the war crimes tribunals.
That continued until the Jan 5 polls.
Security forces started raiding such locations in Satkhira district where violence had taken place, specially areas where minorities had been targetted.
Several Jamaat and some BNP activists have been killed during purported encounters with the security forces.
Lawyers and government officials belonging to Hindu community have received these threat letters.
Manabadhikar Sangha Satkhira has been mentioned as senders on the envelopes.
The language on all the letters was same and they were all hand-written.
The seal on the envelopes indicate they were posted from Khulna.
Several of those who received the letters told bdnews24.com that they would go to the police, seeking security.
OC Enamul told bdnews24.com: "Nobody has yet complained to the police station about this. We are investigating it."