An FBI intelligence bulletin obtained by the Associated Press said those two letters were postmarked Memphis.
Both letters said: "To see a wrong and not expose it, is to become a silent partner to its continuance." Both were signed, "I am KC and I approve this message."
As authorities scurried to investigate three questionable packages discovered in Senate office buildings, reports of suspicious items also came in from at least three senators' offices in their home states.
Sen. Carl Levin said a staff member at his Saginaw, Mich., office would spend the night at a hospital as a precaution after discovering a suspicious letter. The staff member had no symptoms, Levin said in a statement. He expected to learn preliminary results of tests on the letter by Thursday.
Sen. Jeff Flake (R., Ariz.) said suspicious letters at his Phoenix office had been cleared with nothing dangerous found. A package at the Dallas-area office of Sen. John Cornyn (R., Texas) also was declared harmless, a fire department spokesman said.
All three packages in the Capitol complex turned out to be safe, Capitol police spokeswoman Makema Turner said late Wednesday. But a man was still being questioned after being stopped in connection with the packages, she said.
In all, five senators, including some in the thick of contentious negotiations over gun-control and immigration bills, were sent into emergency mode. Another wave of anxiety swept through the Capitol just before lunchtime when a bag left in the entrance of a Senate building brought a bomb squad racing toward Capitol Hill.
After two tense hours, the package was cleared, as were two letters delivered earlier to the offices of Sens. Joe Manchin III (D., W.Va.) and Richard Shelby (R., Ala.).
All the activity came as tensions were high in Washington and across the country after Monday's 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, a week before the marathon.
Obama's press secretary, Jay Carney, said mail sent to the White House was 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.
Capitol Police swiftly ramped up security, and lawmakers and staff were cautioned away from some parts of the Hill complex. After hours of jangled nerves, officials signaled that it was safe to move throughout the area, and people settled back to normal, if watchful, activity.
The Senate sergeant-at-arms, Terrance Gainer, said that an individual who was responsible Tuesday for simultaneous suspicious-package incidents in the Hart and Russell Senate Office Buildings was detained and released Wednesday. The packages were not hazardous.
At a House hearing, Postmaster General Patrick Donahoe noted that there had been ricin alerts since the notorious 2001 anthrax mailings and that procedures were 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."
Even during the flurry of concern, normal business continued across most of the Capitol and its office buildings, with tour groups passing through, and visitors streaming in and out of Wicker's office.
Amy Keough of Stow, Mass., and her family were searching for an open entrance to the Russell Senate Office Building and walked by a U.S. Capitol Police hazardous-materials vehicle. The Keoughs had been visiting Washington for several days, but Monday's 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."
How Deadly Is Ricin?
Ricin is derived from the castor plant, which makes castor oil. What makes it scary is that there is no antidote. It is deadliest when inhaled. It is not contagious.
The Centers for Disease Control and Prevention rates ricin a "Class B" threat, the agency's second-highest threat level - behind anthrax, botulism, plague, smallpox, tularemia, and viral hemorrhagic fevers.
The 2010 Homeland Security Department handbook says that inhaling ricin is more dangerous than eating it but that "formulating ricin powder to produce the necessary size to be efficiently disseminated via aerosol requires technical skill."
SOURCE: Associated Press
This article contains information from the Washington Post.