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“He is bipolar, and the only thing I can say is he wasn’t on his medicine,” his ex-wife, Laura Curtis, told The Associated Press.

Ricin, naturally found in castor beans, can be fatal if ingested, inhaled or absorbed. Ricin poisoning has no known antidote and is difficult to detect as a cause of death.

The letters were among several pieces of suspicious mail reported by senators this week at Capitol and state offices.

Sen. Carl Levin, Michigan Democrat, said Thursday that tests on a suspicious letter sent to one of his state offices showed no signs that it was contaminated with poison.

“I have been advised by the FBI that preliminary testing by the Michigan Department of Community Health, Micro-Biology Laboratory in Lansing (Mich.) showed negative results,” he said. His Saginaw office, which had been closed Wednesday when the letter was discovered, has reopened.

Sen. Jeff Flake, Arizona Republican, said suspicious letters at his Phoenix office had been cleared with nothing dangerous found. A package at Sen. John Cornyn’s Dallas-area office also was declared harmless.

⦁ This article is based in part on wire service reports.