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.
"I'm just tickled to death everybody's safe, that's all I was caring about," said Manchin, who was on the floor of the Senate lobbying for his measure to expand background checks for gun buyers when he was alerted that his staff had been evacuated after a letter had been left on the front desk of his office.
Manchin dismissed the idea that his role in the gun-control legislation, which failed late Wednesday, could have made him a target. Manchin also shook his head at the timing, coming right after the Monday attack in Boston: "Strange time in America, isn't it?"
Late Wednesday, the FBI said it was still awaiting final word on whether the letters to Obama and Sen. Roger Wicker, R-Miss., definitely contained ricin, a poison made of processed castor beans.
Law enforcement officials said the poison was detected in both letters initial screenings Tuesday. They said the two also came up positive in a second, similar test at a military facility in Maryland, the officials said.
More elaborate testing to verify the presence of the poison — using fluorescent antibodies, chromatography and searching for DNA — can take 24 to 48 hours, one law enforcement official said.
The initial tests can be inaccurate. In 2004, a letter to then-Senate Majority Leader Bill Frist , R-Tenn., was believed to contain ricin but later was shown to be harmless.
As a bioterrorism agent, ricin has the advantage of being easily made and highly potent. When castor beans are crushed for oil, the compound is left behind in the mashed material, of which more than a million tons is produced around the world each year.