Mayor Michael Bloomberg was sent two letters that tested positive for traces of ricin — one to his New York office and another to his gun-control group in Washington, the New York Times reported Wednesday.
The toxic substance was also recently found on correspondence sent to US President Barack Obama and several lawmakers late last month.
The anonymous letters were opened Friday in New York and Sunday in Washington, the latter at an office housing Bloomberg's non-profit Mayors Against Illegal Guns.
Police told the Associated Press that people who were exposed to the letters showed no symptoms. However, three New York officers who examined the letter experienced some minor, temporary reactions to the substance.
The Federal Bureau of Investigation is investigating the matter. The mayor had no comment.
Ricin can be deadly if swallowed or inhaled.
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