The letter to Obama contained a "suspicious substance," authorities said.
"A letter addressed to the president was received at a remote screening facility containing a suspicious substance. We are working closely with Capitol Police and the FBI in this investigation," a Secret Service spokesman said.
All White House mail goes through the remote screening facility, officials said. Authorities did not immediately identify the substance in the letter.
The letter sent to Sen. Roger Wicker, a Mississippi Republican, was intercepted at a mail facility Tuesday and did not reach the Capitol. It was postmarked Memphis, Tenn.
The facility used to review all mail sent to Congress is temporarily shut down while a search is under way for the possibility of any other tainted letters.
