A "suspicious substance" has been found inside a letter addressed to President Barack Obama at the White House on Wednesday, according to CNN.
Officials said it is similar to the one sent Tuesday to Mississippi Sen. Roger Wicker. Preliminary tests showed that letter contained the deadly poison ricin, and Capitol police, the FBI and other agencies have launched an investigation.
The letter was sent to a laboratory for further analysis, Capitol police said Tuesday night.
It was postmarked from Memphis, Tenn., and had no return address, said Terrance Gainer, the Senate sergeant at arms, earlier in a warning to members of the Senate.
Gainer first said in a statement that the substance had tested positive for ricin, which is found naturally in castor beans and can cause death from exposure to as little as a pinhead amount, usually within the first 72 hours.
Police later issued a statement saying the test was preliminary and "indicated" that ricin had been found in the letter.
It was intercepted in a mail handling facility and quarantined, the statement said.
"Senate employees should be vigilant in their mail-handling processes for ALL mailings," Gainer said in his written statement.
Members of the Senate were briefed on the incident by Gainer during a Tuesday meeting about the bombings in Boston with FBI Director Robert Mueller and Janet Napolitano, the secretary of Homeland Security.
Several senators told reporters after the briefing that the incident reminded them of the anthrax attacks in the wake of the Sept. 11, 2001, attacks on the Pentagon and World Trade Center.
The suspicious letter 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 Sen. 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 U.S. 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.
A law enforcement source, speaking on condition of anonymity, said initial field tests on the letter produced mixed results, prompting authorities to order further analysis at an accredited laboratory.
It was not immediately clear whether Wicker, a Republican, 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.
There was another ricin scare at the U.S. 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.