Wicker's office could not be reached for comment. But the incident is reminiscent of the weeks following the Sept. 11, 2001, attacks, when a number of letters sent to Capitol Hill offices were found to contain anthrax spores. Anthrax is a rapid-onset disease caused by specific bacteria. Anthrax-tainted letters mailed nationally killed five people, including two postal workers, and sickened 17 others through September and into November that year. Members of the media received envelopes containing anthrax as well.
News of the envelope containing ricin -- a toxic substance found naturally in castor beans, according to the Centers for Disease Control and Prevention -- came as uncertainty lingered over whether the explosions at the Boston Marathon were the product of foreign or domestic terrorism. After intelligence briefings Tuesday, members of Congress were left to hope for an investigative breakthrough.
Gen. James Clapper, director of national intelligence, told Senate Intelligence Committee members that the FBI is taking the lead and that the person of interest that had been questioned was no longer of interest in the case. The FBI provided separate bipartisan briefings House and Senate members.
House Homeland Security Chairman Michael McCaul, R-Texas, said fragments of the bombs that exploded in Boston have been sent to the FBI laboratory in Quantico, Virginia, for forensics and that gunpowder was used.
McCaul's remarks came as he left a closed-door briefing for House members with FBI Director Robert Mueller and National Security Agency Director Keith Alexander on the status of the investigation late Tuesday afternoon.
McCaul said investigators were proceeding along two possible paths for how the devices were detonated in Boston. One was that a cell phone was used to detonate the bomb and another possibility was "a pressure cooker with a timer on it." He added, "The device itself -- the tracers with explosives, the device itself how it's built, the ball bearings -- there's signatures to these things. And they can be traced back to the origin."
A refrain from members who attended briefings on both sides of the Capitol is that law-enforcement and intelligence officials appeared to have had no known warning of any security threats before the Boston attack and there has been no chatter on the airwaves afterward from any groups or individuals taking credit for it.
"We do not have any hard information," Senate Intelligence Chairwoman Dianne Feinstein, D-Calif., said. "The FBI is very good at this, so I have no reason to believe that they won't have a successful investigation that will end in an arrest. But it may well take time, and I think we may need to be patient."
While some members were raising questions about what U.S. officials knew before the attack and what they have learned sin the ongoing investigation, Feinstein and House Intelligence ranking member Dutch Ruppersberger, D-Md., said they did not know of any intelligence that federal officials had to warn of a potential attack and cautioned against criticizing U.S. intelligence. "We've done a very good job of it up to now for 12 years," said Feinstein. "I think there are over 100 potential terrorist attacks that have been stopped because of the excellent work of the FBI in this country and the CIA abroad."
Sen. Saxby Chambliss, R-Ga., ranking member on the Senate Intelligence panel, added that it is "unusual" that no one has claimed credit. "There are a lot of things that are surrounding this that would give an indication that it may have been a domestic terrorist, but that just can't be assumed," he said.
Sen. Tom Coburn, R-Okla., said it was way too soon to assess the administration's handling or response.
Rebecca Kaplan contributed