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An envelope that tested positive for the deadly poison ricin was intercepted Tuesday afternoon at the U.S. Capitol's off-site mail facility in Washington, congressional and law enforcement sources tell CNN.

Senate Majority Leader Harry Reid said he was told the letter was addressed to the office of Sen. Roger Wicker, R-Mississippi. A laboratory in Maryland confirmed the presence of ricin after initial field tests indicated the poison was present, according to Senate Sergeant-at-Arms Terrance Gainer.

However, the FBI said additional testing is needed as field and preliminary tests produce inconsistent results.

"Only a full analysis performed at an accredited laboratory can determine the presence of a biological agent such as ricin," according to the bureau. "Those tests are in the process of being conducted and generally take from 24 to 48 hours."

In a statement late Tuesday, the U.S. Capitol Police said more tests would be conducted at the Army's biomedical research laboratory at Fort Detrick, Maryland.

The letter had a Memphis, Tennessee, postmark and no return address, Gainer wrote in an e-mail to senators and aides.

Sen. Claire McCaskill told reporters after a briefing for lawmakers that a suspect has already been identified in the incident, but a knowledgeable source said no one was in custody Tuesday night.

Wicker, the junior senator from Mississippi, has been assigned a protective detail, according to a law enforcement source.

Postal workers started handling mail at a site off Capitol Hill after the 2001 anthrax attacks that targeted then-Majority Leader Tom Daschle and Vermont Sen. Patrick Leahy, among others. Senators were told the post office would be temporarily shut down "to make sure they get everything squared away," McCaskill, D-Missouri, said Tuesday afternoon.

"The bottom line is, the process we have in place worked," she said. Members will be warning their home-state offices to look out for similar letters, she added.