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WASHINGTON, DC (NBC NEWS) -- The Secret Service is working closely with the FBI after stopping a letter that tested positive for ricin from reaching the White House.

The FBI also says a second mail screening facility may be contaminated, and Senator Carl Levin says his Michigan office received a suspicious letter as well.

Meanwhile a suspicious package outside Senator Richard Shelby's office had the Capitol on alert.

Inside this Senate mail screening facility, police are also searching for additional letters that could be contaminated with ricin.

On Tuesday mail processors intercepted a letter postmarked from Memphis and addressed to Mississippi Senator Roger Wicker.

It tested positive for the potentially deadly toxin during a field test.

The FBI says additional tests are underway to confirm the discovery.

In a statement Senator Wicker thanked law enforcement for their "hard work and diligence".