A tax preparer who sent threatening letters to a Manhattan assistant district attorney who had twice prosecuted him was sentenced Friday to three to six years behind bars.

Prosecutors arrested Jack Chang, 55, last summer after Gilda Mariani, the chief of the money laundering and tax crime unit in the district attorney’s office, received two ominous letters. One was addressed to her husband at her home and contained a white powder that turned out to be corn starch. The other was delivered through interoffice mail.