WASHINGTON (AFP) - Federal authorities intercepted a letter bound for the Washington office of US Senator Roger Wicker that contained the deadly poison ricin, a Senate leadership aide said.

The letter was detected during a routine inspection of mail at an off-site facility on Tuesday and did not reach the US Capitol or Senator Wicker's office, the aide said, citing Senate Majority Leader Harry Reid, who discussed the security breach with lawmakers.

The letter "tested positive for ricin," the aide told reporters.

FBI Director Robert Mueller and Homeland Security Secretary Janet Napolitano told senators about the incident during a closed-door briefing on Tuesday evening on the terrorist attacks in Boston, the official said.

It was not clear whether there was a connection between the Boston attacks and the ricin discovery.

Congressional mail has been screened off-site since letters laced with anthrax were sent to Capitol Hill in 2001.

The aide gave no indication why the latest letter was sent to Senator Wicker.

Three Senate office buildings were shut in 2004 after tests found ricin in mail that had been sent to the Senate majority leader's office.

Aides to Senator Wicker, a Republican senator from Mississippi, did not immediately respond to requests for comment.