Last month, ricin-tainted letters were sent to President Obama and to government offices, including that of Sen. Roger Wicker, R-Miss.
The letter addressed to the president was received at the White House mail facility and included the message "To see a wrong and not expose it is to become a silent partner to its continuance."
The Senate mail facility received the letter addressed to Wicker. It contained the same message.
A ranking counter-terrorism official in New York marveled at how ricin has become so popular among those trying to scare of harm others more than 11 years after anthrax-tainted mail terrified a nation still reeling from the 9/11 attacks.
"Instead of the drug of choice, it [ricin] has become the weapon of choice of people who might be thinking that," the official said.
One Bloomberg letter was opened by screeners at the City Hall mail facility a few blocks from the building on Gold Street, in Lower Manhattan. Bloomberg's aides said the episode proves the counter-terror measures in place at City Hall worked because no unscreened or suspicious mail gets to the mayor, his staff or anyone else who works in the building – including the press corps.
The second letter was sent to Mark Glaze, director of Bloomberg's group, which has its offices in Northwest Washington, near the White House. Glaze actually opened the letter himself on a park bench and then called for help after seeing the powder in the envelope, a source told ABC News. He declined to comment.
A D.C. police report obtained by ABC News said Glaze found the envelope to contain "a threatening message which had a whitish orange substance on the note…The letter was addressed to [Glaze] but there was no return address or sender's name."
ABC News' Josh Haskell contributed to this report.