Letter to Obama Had Ricin; Senate Building Evacuated

A letter containing a "suspicious substance" was sent to President Obama and intercepted at the White House's off-site mail center, according to the Secret Service, and the FBI has confirmed that it has tested positive for ricin. It arrived yesterday, the same day as a letter addressed to a Mississippi Republican Sen. Roger Wicker that also tested positive for the poison, CNN reports. Both letters were postmarked out of Memphis April 8, and both say: "To see a wrong and not expose it, is to become a silent partner to its continuance." They are signed, "I am KC and I approve this message," the AP reports. Officials think they know who sent the letters (Claire McCaskill said yesterday that police had a suspect in mind), but no arrest has been made yet. Related developments:

• A second letter sent to the US Senate has also been intercepted and is being tested for the poison, the AP reports; Terry Moran of ABC News tweets that that second letter also tested positive.
• Sen. Carl Levin says a suspicious letter arrived at one of his Michigan offices, but it's not yet known if it presented any threat.
• Also today, Capitol police investigated a suspicious package at Sen. Richard Shelby's office, NBC News reports. The AP reports that two suspicious packages were being investigated at two different Senate office buildings. The first floor of one building was evacuated, CNN reports, and police questioned a suspicious man with a backpack seen in the area. Envelopes found inside his backpack were being X-rayed. CNN tweets that the packages were removed and the buildings reopened.
• At this point, the FBI does not think the letters are related to the Boston Marathon bombing, an agency official says.

(More ricin stories.)