The FBI has announced that an envelope addressed to the White House has tested positive for ricin, and has been quarantined by US Secret Service personnel. A coordinated investigation has since been initiated by the FBI.
Although the letter initially tested positive for the deadly poison, official test results will be available in 24 to 48 hours. According to a law enforcement official, preliminary tests are "unreliable" and often result in false positives.
In an intelligence bulletin obtained by AP, the FBI says that both letters say: "To see a wrong and not expose it, is to become a silent partner to its continuance." Both are signed, "I am KC and I approve this message."
Federal officials say they believe they know who sent the letters, but that no arrests have been made.
"A letter addressed to the president was received at a remote screening facility containing a suspicious substance. We are working closely with Capitol Police and the FBI in this investigation," a Secret Service spokesman said in a statement.
The letter, which was received at an off-site location away from the White House on Tuesday, is currently under investigation after being intercepted at a postal screening center.
"This facility routinely identifies letters or parcels that require secondary screening or scientific testing before delivery," White House spokesman Edwin Donovan said in a statement.
It arrived the same day as an envelope addressed to US Senator Roger Wicker, which tested positive for ricin. The envelope was postmarked Memphis, Tennessee and did not have a return address. The newest letter is believed to be from the same sender.
The FBI says that it has not found a link between the two letters and Monday's attack at the Boston Marathon.
Meanwhile, Arizona Senator Jeff Flake has received two suspicious letters at his Phoenix office, AP reported.
Emergency crews have evacuated people from parts of the building while police investigate whether the packages are dangerous.
Michigan Senator Carl Levin and Alabama Senator Richard Shelby have reportedly received suspicious letters, as well.
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