UPDATED at 7:30 p.m.
Two federal officials tell NBC News that agents have arrested a man from Tupelo, Miss., as a suspect in the mailing of the intercepted letters that initially tested positive for ricin.
A joint FBI/Secret Service investigation is underway after a letter addressed to President Obama was intercepted and initially tested positive for ricin on Tuesday.
The letter was caught at a remote Secret Service White House mail screening facility, not located near the White House complex. The president has been briefed on these letters, White House Press Secretary Jay Carney said on Wednesday.
Authorities cleared the atrium of a Senate building due to another suspicious package on Wednesday. Capitol Police are also investigating a suspicious package at Sen. Richard Shelby’s office in the Russell Senate Office building on Capitol Hill. Capitol Police later alerted Senate staffers that suspicious envelopes and a package had been removed from both buildings, and they are now being allowed to leave their offices.
Authorities have already intercepted a separate letter sent to Mississippi Republican Sen. Roger Wicker. The tainted letter was red-flagged through an off-site screening process at a U.S. Post Office in Landover, Md., sources told NBC News. The envelope tested positive for Ricin, a biotoxin that can be fatal even in small doses and is derived from the castor bean plant.








