Letters sent to President Barack Obama and a Mississippi senator have tested positive for poisonous ricin in preliminary checks, and authorities chased reports of other suspicious mail at a US Capitol already on edge.
An FBI intelligence bulletin said the letters to Obama and Senator Roger Wicker were postmarked Memphis, Tennessee.
Both letters say: "To see a wrong and not expose it, is to become a silent partner to its continuance." Both are signed, "I am KC and I approve this message".
Those letters were intercepted before reaching the White House or Senate. The FBI said more testing was underway. Preliminary tests often show false positives for ricin.
As authorities scurried to investigate at least three questionable packages discovered in Senate office buildings, reports of suspicious letters also came in from senators' state offices in Arizona and Michigan.
Staff members were cautioned away from parts of the Capitol complex as police ramped up security.
After a couple of mid-day hours of jangled nerves, Capitol officials signaled it was safe to move throughout the complex and lawmakers and their staff settled back to normal, if watchful, activity.
There was no immediate information from Capitol police about the contents of the suspicious packages.
The activity came as tensions were high in Washington and across the country following Monday's twin bombings at the Boston Marathon that killed three people and injured more than 170. The FBI said there was no indication of a connection between the letters and the bombing. The letters to Obama and Wicker were postmarked April 8, before the marathon.
Senator Claire McCaskill, said that police have a suspect in mind in the Wicker mailing, someone who "writes a lot of letters to members". She made the comment Tuesday as she emerged from a briefing by law enforcement on the Boston bombing. Authorities declined to comment on a possible suspect.
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 ricin testing, referring questions to the FBI.
At a House hearing, Postmaster General Patrick Donahoe said there have been ricin scares since the 2001 anthrax mailings and procedures are in place to protect postal employees and government offices.
"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.
Despite the increased police presence in some areas, and some staffers cautioned to stay in their offices, normal business continued across most of the Capitol and its office buildings, with tour groups passing through as usual and visitors streaming in and out of Wicker's office.
Senate Sergeant-at-Arms Terence Gainer said in an email that suspicious packages were dropped off at the offices of two senators. Senator Richard Shelby, 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 Capitol police swiftly ramped up security and temporarily advised people to avoid parts of the office buildings.
Both the letters to Wicker, and to Obama were intercepted at off-site mail facilities. The FBI said the letters were undergoing further, more conclusive testing. Ricin, derived from the castor plant, is at its deadliest when inhaled.
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 Senator 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 US Capitol Police hazardous materials vehicle. The Keoughs have 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."
Senator Carl Levin issued a statement saying an aide in his office had received a suspicious-looking letter. "The letter was not opened, and the staffer followed the proper protocols for the situation, including alerting the authorities, who are now investigating," the Michigan Democrat said in a statement.
Authorities also are investigating two suspicious letters that were sent to the Phoenix office of Senator Jeff Flake, police said. Emergency crews in hazardous materials gear were seen outside the building.
The Centers for Disease Control and Prevention said there's a rapid detection test for ricin that takes 6 to 8 hours, but the more complete test - the ricin toxin test - takes about 48 hours to perform.