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."
Associated Press writers contributing to this report from Washington were: Eileen Sullivan, Laurie Kellman, Donna Cassata, Henry Jackson and Eric Tucker.