US authorities intercepted a letter that contained the deadly poison ricin and was bound for Senator Roger Wicker's office, officials said Tuesday, triggering new security concerns a day after the terror attack in Boston.

The tainted letter was detected during a routine mail inspection at an off-site facility and did not reach the US Capitol or Wicker's Washington office, a Senate aide said, citing Senate Majority Leader Harry Reid. Senate Sergeant at Arms Terrance Gainer broke the news to the chamber's members, saying in a statement that "the Senate mail handling facility that services members' DC offices has received mail that tested positive for ricin."

Federal Bureau of Investigation Director Robert Mueller and Homeland Security Secretary Janet Napolitano briefed senators about the incident during a closed-door meeting Tuesday evening on the Boston bombings, the Senate aide told reporters.

Wicker released a brief statement saying the matter is under investigation by the FBI and US Capitol Police.

"I want to thank our law enforcement officials for their hard work and diligence in keeping those of us who work in the Capitol complex safe," he said. "Gayle and I appreciate everyone's thoughts and prayers."

Officials gave no indication why the letter was sent to Wicker, a Republican two-term Senator from the southern state of Mississippi.

They also made no mention of whether anybody fell ill during the incident.

The SITE monitoring service reported overnight Wednesday that Wicker was the target of anger from US-based militia groups and white supremacists for his vote that helped move forward gun reform debates in the Senate.

One poster to an online forum wrote "Traitorous dirtbags...TREASON! Every one of you communist, liberal, anti-white, anti-American filth that voted in favor of this constitution-trashing motion should HANG from the HIGHEST LIMB publicly, as an example to all other would-be traitors to our once-great country. FILTH"

In another thread, a poster said "organization is going on behind the scenes," in response to someone who lamented a lack of planning "for what needs to take place here."

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 Monday's bombings near the finish line of the Boston Marathon gave added resonance to the positive ricin test.

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

Lawmakers on the other side of the US Capitol, in the House of Representatives, were also informed of the breach.

"It is imperative that if you are opening mail, that you take precautions as well as to be vigilant," the House Sergeant at Arms office said in an email to congress members and their staffs.

"It is important to note that at this time, the House mail handling facility has not received any suspicious mail," it said.

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

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.

Gainer said the exterior markings on the latest 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 minuscule quantities.

Last week the Senate, including 16 Republicans agreed to debate what has emerged as the most significant gun legislation in nearly 20 years. But passing new gun laws is controversial in the US, where many say they would infringe on the constitutional right to bear arms.