This is a breaking news story. Check back for updates.
Update 1:26 p.m.: The ricin-laced letters sent to President Obama and Mississippi Sen. Roger Wicker contain similarities, including the same phrase and the same signature, CNN reports. The New York Post reported that both letters had a Memphis postmark and were dated April 5.
"To see a wrong and not expose it, is to become a silent partner to its continuance," both letters said. They were also both signed off with, "I am KC and I approve this message."
Update 1:16 p.m.: Authorities have cleared restricted areas in the Senate office buildings where the suspicious packages were reported, according to CNN. U.S. Sen. Carl Levin, D-Mich., also said a suspicious-looking package was received in one of his district offices.
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Police: Restricted areas at 2 Senate office buildings reopened; suspicious items removed. on.cnn.com/112WXpr
â€” CNN Breaking News (@cnnbrk) April 17, 2013
Update 12:43 p.m.: The FBI says so far there is no indication that the suspicious letters are related to Monday's explosions at the Boston marathon.
"The investigation into these letters remains ongoing, and more letters may still be received. There is no indication of a connection to the attack in Boston," the agency said Wednesday in a statement.
A man believed to have addressed a suspicious letter to President Obama is in custody, a day after a ricin-laced letter addressed to a U.S. senator was intercepted, according to CNN.
The letter addressed to the president was intercepted Tuesday by the U.S. Secret Service at the White House mail processing facility set up following the 2001 anthrax attacks that targeted Washington lawmakers, according to the FBI.
"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 envelope, addressed to the White House, was immediately quarantined by U.S. Secret Service personnel, and a coordinated investigation with the FBI was initiated. It is important to note that operations at the White House have not been affected as a result of the investigation."
The man was described as “disturbed,” according to CNN. The letter may have been tainted with ricin, the same toxic substance found in the letter addressed to Mississippi Sen. Roger Wicker, a Republican, the Associated Press reported.
Meanwhile, U.S. Capitol Police were responding to reports Wednesday of suspicious packages at the Hart and Russell Senate office buildings. Staffers and lawmakers were told to stay in their offices as police investigate the packages.