Federal prosecutors charged Matthew Unglesbee, 36, of Quincy, Illinois, with mailing threatening letters that held a white granular substance. NBC News and the Washington Examiner reported the Monday case. He is accused of sending four letters to members of Congress with his name and home address on the envelopes.

The FBI opened the case after an anonymous tipster said they overheard the man talking about mailing a suspicious substance to Congress. Agents later matched letters that arrived at congressional offices. Lab tests on the powder were not fully detailed on the first-day pages.

Unglesbee faces charges tied to false information and hoaxes and mailing threatening communications. He appeared in court Monday. A detention hearing was set for Wednesday afternoon in Springfield.

Members of Congress have faced powder letters before, and each case triggers hazmat checks. Officials said the substance did not appear to be a live toxin in early screening, but that claim was not final on every page. Prosecutors said the letters were meant to intimidate.

What we know: Unglesbee is charged in the white powder letter case and has a detention hearing Wednesday.
What we don't: Final lab results on every sample, and whether more letters were sent.