An Illinois man was charged with two felonies Monday after allegedly sending letters containing an unknown white powder substance to members of Congress, according to a criminal complaint.

Matthew G. Unglesbee was charged with false information and hoaxes and mailing threatening communications after four members of Congress received the letters, which were marked with his return address, according to the complaint.

Unglesbee told the FBI the substance in the letters was salt and that he mailed them as a joke, the complaint read.

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The FBI received a tip that Unglesbee had sent the letters on Oct. 16. The tipster had overheard Unglesbee at a restaurant saying he had mailed suspicious substances to members of Congress, the complaint stated.

The tipster allegedly provided authorities with Unglesbee’s full name, address and date of birth.

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Members of the FBI’s field office in Chicago responded to the offices of three members of Congress in the northern Illinois area after receiving the tip and a 911 call from one of the congressional offices.

"A staff member at Congressperson A's office, the first office to receive a letter, noticed that the letter felt like it contained a powder and the staffer could hear something inside the letter," the complaint read. This staff member called 911.

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The office had already been on high alert, having received a similar letter within the previous few weeks, according to the complaint.

All three letters contained "a white granular substance" and a handwritten letter which read "HA HA," the complaint stated.

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On Friday, FBI Chicago responded to the office of a fourth member of Congress, a senator in the Northern District of Illinois. The senator had also received an envelope with a white granular substance, also with a letter which also read, "HAH enjoy the gift."

All four letters were marked with Unglesbee’s name and return address, according to the complaint.

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FBI Task Force Officer Chris Hobbs, who wrote the criminal complaint, characterized the letters as a common intimidation tactic, pointing out that preliminary lab testing "did not detect the presence of chemical or biological toxins in the white granular substance."

The FBI interviewed Unglesbee at his home Saturday when he allegedly confessed to sending the letters, telling law enforcement that he heard someone in a restaurant talking about similar letters Congress members had received.

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He told the FBI he knew his powder would be perceived as potentially being anthrax and anticipated people responding with hazmat suits.

He also told the FBI that the powder was salt, claiming he used salt specifically because it looked like anthrax.

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"Unglesbee claimed he was joking but recognized that the recipients would not know he was joking and could perceive it as a threat," the complaint read.

The FBI searched Unglesbee's home and found a fifth letter marked with one of the congresspeople's Washington, D.C., office's address.

The FBI also determined that Unglesbee sent the letters on Oct. 19.

Fox News Digital contacted the FBI for further information.