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UPDATE: Secret Service official: Letters test positive for Ricin

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WASHINGTON (NBC) -- A letter addressed to President Barack Obama tested positive for the poison ricin and was from the same sender who mailed a letter to a senator that also tested positive, officials told NBC News on Wednesday.

The letter to Obama was intercepted at an off-site White House mail facility and was being tested further, a Secret Service official told NBC News. A federal law enforcement official said that the letter was "very similar" to one addressed to a senator that tested positive in preliminary screening for ricin.

Authorities cleared the atrium of a Senate office building Wednesday, but it was not immediately clear why. Capitol police were also investigating a suspicious package at the office of Sen. Richard Shelby, R-Ala. Shelby's staff had not been evacuated.

Federal officials told NBC News that they believe they know who sent the letters, but no arrest was made, and authorities were waiting for further test results.

Ricin is made from castor beans and can kill within 36 hours. There is no antidote. Some threatening letters simply contain ground castor beans, resulting in a positive field test for ricin without the concentrated poison.

An FBI official told NBC News that the agency did not initially believe the letters were related to the attack on the Boston Marathon on Monday.

The letter to Obama was received at the mail facility Tuesday, the official said. The FBI said Tuesday that a letter intended for Sen. Roger Wicker, R-Miss., had initially tested positive for ricin. Further tests were being conducted.

The Wicker letter was intercepted at a postal facility in Maryland that screens mail sent to Congress, and never reached Wicker's office. The senator thanked law enforcement and said an investigation was underway but did not elaborate.

Other senators were made aware of the Wicker letter during a briefing Tuesday evening on the bombing in Boston. Sen. Claire McCaskill, D-Mo., said that the person who sent Wicker the letter writes often to elected officials.

The Wicker letter was postmarked Memphis, Tenn., and had no return address.

Pete Williams, Kasie Hunt, Kelly O'Donnell and Richard Esposito of NBC News contributed to this report.

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