The letter was postmarked from Memphis, Tennessee, and had no return address, Terrance Gaines, the Senate sergeant at arms, said in a warning to members of the Senate.
"Senate employees should be vigilant in their mail handling processes for ALL mailings," Gaines said in a written statement.
Members of the Senate were briefed on the ricin incident by Gaines during a meeting with FBI Director Robert Mueller and Janet Napolitano, the secretary of Homeland Security, on Tuesday on the bombings in Boston.
Several senators told reporters after the briefing that incident reminded them of the anthrax attacks in the wake of the September 11, 2001, attacks on the Pentagon and World Trade Center.
The positive ricin test came one day after the explosions at the Boston Marathon that killed three people and injured 176.
"I don't know if it's a coincidence. It's too early to tell. We don't know enough about Boston," said Senator Richard Durbin, the No. 2 Democrat in the Senate.
Senate Majority Leader Harry Reid said he had been told the letter was addressed to Wicker, a Republican senator from Mississippi.
All mail to the US Senate had been stopped, and post offices at the Capitol had been closed as a precaution, the senators said. They were getting in touch with their state offices, where mail is not subject to the same extensive screening, to ensure that precautions were being put in place.
Many senators expressed concern about their staffs and the risks to postal workers.
They said they were aware of only one letter that had been intercepted that tested positive for ricin.
It was not immediately clear whether Wicker had attended the briefing by Mueller and Napolitano that was open to all senators from both parties.
Wicker issued a statement saying only that the matter was being investigated and expressing gratitude for thoughts and prayers on his behalf.
"This matter is part of an ongoing investigation by the United States Capitol Police and FBI. 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 in the statement.
PREVENTION SYSTEM WORKED
Wicker, a former member of the House of Representatives, has been a member of the Senate since he was first appointed to a vacant seat in December 2007. He won a special election to serve the remainder of that term, and was re-elected in November 2012 to a full six-year term.
Several senators noted that the system of mail screening, begun after the anthrax attacks, had worked.
"The bottom line is the process we have in place worked," said Claire McCaskill, a Democratic senator from Missouri. She said a suspect had been identified, and said it was someone who wrote to senators often.
Other officials could not immediately confirm that report.
Ricin is a lethal poison found naturally in castor beans. It can cause death from exposure to as little as a pinhead amount. Most victims die between 36 hours and 72 hours after exposure.
There was another ricin scare at the US Capitol in 2004, when tests identified a letter in a Senate mail room that served the office of Bill Frist, a Tennessee Republican who was then the Senate Majority Leader.
The most famous case of ricin poisoning was in 1978 when dissident Bulgarian writer Georgi Markov was killed after a passerby in London jabbed him with an umbrella that injected a tiny ricin-filled pellet.
In 2001, the Capitol was one target in a series of anthrax attacks that killed at least five people on the East Coast, including two Washington postal workers.
Letters containing anthrax spores were mailed to the Washington offices of two senators and to news media outlets in New York and Florida.