(WASHINGTON) -- A letter addressed to President Obama containing a suspicious substance was intercepted on Tuesday, the Secret Service tells ABC News.
The letter was received at a remote mail screening facility not located near the White House complex.
Officials tell ABC News the letter tested positive in a field test for ricin. However, they emphasized that these tests are preliminary and unreliable. A final and complete analysis will be done at an accredited lab.
The Secret Service is investigating the matter, working closely with the U.S. Capitol Police and the FBI.
On Tuesday, the office of Sen. Roger Wicker of Mississippi also received a suspicious letter potentially laced with ricin, postmarked from Memphis. Testing on that letter is incomplete, but expected to be finished Wednesday afternoon.
Meanwhile, Senate offices were on partial lockdown Wednesday after the discovery of suspicious packages. The police investigation centered on the offices of Sens. Richard Shelby, R-Ala., in the Russell Senate Building, and Joe Manchin, D-W.Va, in the Hart Senate Office Building.
The lockdown was unrelated to the Wicker letter.
Several senators, including Carl Levin, D-Mich., have reported suspicious packages delivered to their district offices, with no reports of any credible threats.
But underscoring the jitters among Senate D.C. and district offices, Sen. Deb Fischer's, R-Neb., Lincoln office contacted police when staff found a suspicious package outside this morning.
It turned out to be a used-car part left in a bag on top of a lawn chair.
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