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.
Federal officials said they believe they know who sent the letters to both Wicker and Obama, but no arrests have been made. Although both letters initially tested positive for ricin, authorities won’t be able to confirm the toxin was used until they receive lab results.
The FBI conducts filed tests anytime a suspicious powder is found in a mail facility, officials say. But they warn preliminary tests can produce inconsistent results. A full lab report is expected within the next day or two, officials say. The current assessment is that there is no link between the letters and the bombings in Boston.
Meanwhile, Democratic Sen. Carl Levin also said his regional office in Michigan also received a suspicious-looking letter.
"Earlier today, a staffer at my Saginaw regional office received a suspicious-looking letter. The letter was not opened, and the staffer followed the proper protocols for the situation, including alerting the authorities, who are now investigating. We do not know yet if the mail presented a threat. I'm grateful for my staff's quick response and for government personnel at all levels who are responding."