Ricin can be fatal if ingested or inhaled. In 2004, Senate offices were closed for days after the poison was found in the mailroom of Senator Bill Frist, a Tennessee Republican, who was the majority leader then. And detection of ricin carried echoes of the anthrax attack on the Senate just days after the terrorist attacks of Sept. 11, 2001.

Senator Claire McCaskill, Democrat of Missouri, told reporters after the Napolitano briefing that the letter had come from someone who frequently writes lawmakers. She said the person had been identified, but she declined to divulge the name.

Senator Jeff Flake, Republican of Arizona, said: “Obviously, I’m concerned. It’s not just members concerned. It rarely gets to a member before it goes through a lot of staff, and that’s a big concern, obviously, for all of us. We’re very anxious to get more details.”

As a precaution, Senate post offices on Capitol Hill were shut down and will most likely remain closed for the rest of the week. Senators were told that there was no evidence of contamination inside the Capitol or on the grounds, but investigators were still looking into whether any similar envelopes had been sent to anyone in the House.

As Senator Angus King left Ms. Napolitano’s briefing, he said, “It’s now been confirmed that ricin was found in an envelope at the screening facility, which is off-site, not here at the Capitol.”