A Palm Beach County jail inmate has been charged with making fake anthrax threats after sending letters containing a powdery substance and shutting down the courthouse and nearby Florida state attorney's office, authorities said Thursday.

Anthony Vassalotti, 40, sent the letters to an assistant state attorney who had recently prosecuted him, to the judge who presided over the case, and to the court clerk, according to the U.S. Attorney's Office for the Southern District of Florida.

The letters, which also included Vassalotti's name and return address, were received on Monday. The powder was later found to be harmless. Vassalotti was convicted March 8 of aggravated battery with a firearm, among other charges, and was sentenced to 40 years in prison.

If convicted in the anthrax hoax case, he faces up to 15 years in federal prison.