A letter mailed to President Obama that is similar in some way to two possibly ricin-laced letters sent to New York City Mayor Michael Bloomberg was intercepted Thursday at a mail-handling facility, the Secret Service and other law enforcement authorities confirm.

Secret Service spokesman Brian Leary says in an email to NPR that:

"The White House mail screening facility intercepted a letter addressed to the White House that was similar to letters previously addressed to Mayor Bloomberg in New York. This letter has been turned over to the FBI Joint Terrorism Task Force for testing and investigation"

NPR's Dina Temple-Raston reports hearing the same from other law enforcement sources.

About those letters to Bloomberg, we wrote Wednesday that:

"The anonymous letters ... were opened Friday in New York at the city's mail facility and Sunday in Washington, D.C., at the headquarters of the nonprofit started by Bloomberg, Mayors Against Illegal Guns, according to authorities.

"Reuters, quoting a police statement, reports that both letters 'contained threats against Bloomberg and mentioned the gun debate.' "

These latest reports follow the news from April about letters sent to the White House, a senator and a Mississippi state judge that tested positive for ricin, and the news from last week that a Washington state man was under arrest in connection with two possibly ricin-tained letters sent to officials in that state.

A related post from the Shots blog: "How Ricin Can Sicken And Kill."

Update at 5:40 p.m. ET. Letters Postmarked In Louisiana:

Dina reports that the letters to the president and the New York mayor were both postmarked in Louisiana.

She notes that while "preliminary tests found traces of ricin ... early tests are often false positive." More definitive tests could take days to complete, she says.

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