UPDATE: An arrest has been made. From the New York Times:
WASHINGTON ““ Federal agents arrested a man on Wednesday who is suspected of sending letters believed contaminated by the poison ricin to President Obama and a Republican senator. The F.B.I. identified the suspect as Paul Kevin Curtis of Corinth, Miss.
FEDERAL LAW enforcement officials have confirmed to NBC that a “suspicious substance” on a letter was sent to President Obama that is “similar” to Sen. Wicker earlier this week. Both are currently being tested for ricin.
White House mail handlers identified a “suspicious substance” in a letter sent to President Barack Obama the same day one suspected of containing the poison ricin was found in a Senate mailroom, the Secret Service said Wednesday.
Both letters arrived Tuesday at off-site postal facilities set up after the 2001 anthrax attacks and have been sent to laboratories for additional tests, authorities said.
“A letter addressed to the president containing a suspicious substance was received at the remote White House mail screening facility,” Secret Service spokesman Brian Leary said. The Secret Service, FBI and Capitol Police are investigating, he said.
According to NBC’s Pete Williams, because mail isn’t sent directly to congressional offices or the White House, no one personally received these letters. Williams also reports that authorities believe they know who sent the letters, which are being tested for possible poison, though nothing as strong as anthrax appears to be involved.
There is no connection between these events and the Boston marathon terrorist attack at this point.