Federal authorities intercepted a letter bound for the Washington office of U.S. Senator Roger Wicker that contained the deadly poison ricin, a Senate leadership aide said Tuesday, according to AFP.
The letter was detected during a routine inspection of mail at an off-site facility and did not reach the U.S. Capitol or 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, reported AFP.
FBI Director Robert Mueller and Homeland Security Secretary Janet Napolitano told senators about the incident during a closed-door briefing 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. Napolitano indicated on Tuesday that there is no evidence that the bombings at the Boston Marathon are part of a wider plot.
At the same time, she urged the American public to remain vigilant and to immediately report any signs of suspicious activity to local law enforcement officials.
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 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 Wicker, a Republican senator from Mississippi, did not immediately respond to requests for comment.