Obama's press secretary, Jay Carney, said mail sent to the White House is screened at a remote site for the safety of the recipients and the general public. He declined to comment on the significance of the preliminary ricin result, referring questions to the FBI.
At a House hearing, Postmaster General Patrick Donahoe noted there had been ricin alerts since the notorious 2001 anthrax mailings and procedures are in place to protect postal employees and help track down culprits.
"Over the course of years we've had some situations where there have been ricin scares,'' Donahoe said. "Until this date, there's never been any actually proved that have gone through the system.''
After the hearing, Donahoe said he didn't know whether the latest letters had been proven to contain ricin. He also told reporters that people sometimes mail substances that mimic the poison. No postal workers have reported illness connected to the incident, he said. Ricin, derived from the castor plant, is at its deadliest when inhaled.
Even during the flurry of concern, normal business continued across most of the Capitol and its office buildings, with tour groups passing through and visitors streaming in and out of Wicker's office.
Senate Sergeant-at-Arms Terrance Gainer said in an email that suspicious packages were dropped off at the offices of two senators. Sen. Richard Shelby, R-Ala., said in a statement his office had received one of them.
A third package was found in an atrium on the first floor of a Senate building. A man who delivered at least two of the packages was being questioned, Gainer said.
As the discoveries spread concern, police sealed off a hearing room where Defense Secretary Chuck Hagel and Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, were testifying. At one point, officers advised Sen. Joe Manchin and aides not to board an elevator because suspicious packages had been found on several floors of the Hart Office Building. "They just told me there's something suspicious and they're looking into it,'' Manchin said.
Amy Keough of Stow, Mass., and her family were searching for an open entrance to the Russell Senate Office building and walked by a U.S. Capitol Police hazardous materials vehicle. The Keoughs had been visiting Washington for several days, but Monday's marathon bombing was on their minds.
"We don't know really what it is that's going on,'' Keough said. "We're from Massachusetts, so right now anything is possible, with all the events in Boston.''