WASHINGTON (AFP) ― U.S. federal authorities intercepted a letter that contained the deadly poison ricin and was bound for the Washington office of Senator Roger Wicker, officials said on Tuesday.

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

“The Senate mail handling facility that services members’ D.C. offices has received mail that tested positive for ricin,” Senate Sergeant at Arms Terrance Gainer said in a statement.

“The United States Capitol Police, the FBI and other agencies are involved in the investigation of this mailing,” he added.

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 Senate aide said.

It was not clear whether there was a connection between the Boston attacks and the ricin discovery, but the heightened security concerns in the wake of the attacks gave added resonance to the positive ricin test.

Gainer said “while we have no indication that there are other suspect mailings, it is imperative to follow all mail handling protocols.”

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

Officials 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.

The biological agent was also sent to the White House and the Department of Transportation in November 2003. There were no injuries in those incidents.

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

Gainer said the exterior markings on the envelope “were not outwardly suspicious, but it was postmarked from Memphis, Tennessee and had no return address.”

“These incidents are reminders that we need to remain vigilant in handling mail, recognizing suspicious items, and knowing what immediate actions to employ if faced with suspicious mail in the office,” the sergeant at arms said.

The Senate mail facility will be closed “for the next two to three days while testing and the law enforcement investigation continues,” he added.

Ricin when inhaled can cause respiratory problems. Ingested orally, the protein is lethal in even miniscule quantities.