CNN has been wrong so often lately, I'm not sure whether or not to share this
It's been confirmed by at least one other source.
UPDATE II: Looks like a suspect may already be in custody - h/t ericlewis0
Landrieu told reporters that the senators were told the suspected letter writer writes a lot of letters to members and added she believes authorities have someone in custody.
As a precaution, the Senate post office has been temporarily shut down.
UPDATE: Senate Majority Leader Harry Reid said he was told the letter was addressed to the office of Sen. Roger Wicker, R-Mississippi.
Envelope tests positive for ricin at Washington mail facility
By Mike Brooks and Dana Bash, CNN
April 16, 2013 -- Updated 2230 GMT (0630 HKT)
Washington (CNN) -- An envelope that tested positive for the deadly poison ricin was intercepted Tuesday afternoon at the U.S. Capitol's off-site mail facility in Washington, congressional and law enforcement sources tell CNN.
After the envelope tested positive in a first routine test, it was retested two more times, each time coming up positive, the law enforcement source said. The package was then sent to a Maryland lab for further testing.
The envelope was addressed to a U.S. senator, though the sources would not say which one.