(CBS) – The man accused of sending a letter to the White House – a letter that tested positive for cyanide – is from Chicago.

The letter never made it to the president’s residence. It was discovered Monday at a facility that screens mail.

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It contained a milky white substance that tested negative at first for cyanide, and then positive during a second test. It’s being tested a third time.

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The suspect is described as a homeless man from Chicago.

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Officials think he suffers from mental illness because of how the letter was written.