An Illinois man is facing federal charges related to mailing letters containing a white substance to four members of Congress this month in an act inspired by reports of suspicious packages being sent to dozens of lawmakers and election officials across the country.

Matthew Unglesbee, 37, is charged with one count of false information and hoaxes and one count of mailing threatening communications, according to a criminal complaint unsealed Monday. He made an initial appearance in federal court via video Monday afternoon.

An accompanying affidavit states that FBI personnel responded to three congressional members’ offices in the Northern District of Illinois last Wednesday after receiving an anonymous tip. Letters containing a “white granular substance" — later determined to be salt — were found along with handwritten notes that read “HA HA,” according to the affidavit. A fourth envelope was also found at a U.S. senator’s office with the message: “HAH enjoy the gift.”The envelopes identified Unglesbee as the sender and included his home address, the FBI said.

Court documents indicate that Unglesbee told FBI agents Sunday that he sent the letters. He claimed that he was “joking” but knew it could be perceived as a threat. The document states he said he chose salt because it “kind of looks like” anthrax, which he knew recipients took seriously and would have “hazmat come in white suits.”

A fifth unmailed, empty envelope addressed to a congressional office in D.C. was also found during a search of Unglesbee’s home, according to the documents.

Dozens of congressional offices from both parties have reported receiving mail containing an unknown white powder in recent weeks, prompting a federal investigation. Unglesbee has denied any involvement in those other incidents.