Washington (CNN) — 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. After the envelope tested positive in a first routine test, it was retested two more times, each time coming up positive, the law enforcement source said. The package was then sent to a Maryland lab for further testing.

Sen. Claire McCaskill told reporters after a briefing for lawmakers that a suspect has already been identified in the incident. Members will be warning their home-state offices to look out for similar letters, she said.

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 those offices would be temporarily shut down “to make sure they get everything squared away,” McCaskill, D-Missouri, said Tuesday afternoon.

Envelope tests positive for ricin at Washington mail facility-CNN