The Wall Street Journal has confirmed the news with the NYPD.
Bloomberg never personally received the poison letter.
Another letter that also tested positive for the substance was reportedly sent to a Washington, D.C. building that houses Mayors Against Illegal Guns, which Bloomberg helps run.
The contents of the letter reportedly threatened Bloomberg and referenced the debate on gun laws, according to a local ABC news station.
The letters apparently came from Shreveport, Louisiana, according to a statement on the American Postal Workers Union website.
Authorities are now " in the process of searching to determine if there may be additional letters," according to the statement.
Last month, letters sent to President Barack Obama and an office of Sen. Roger Wicker (R-Miss.) tested positive for ricin.
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