Mail for members of Congress and the White House has been handled at off-site postal facilities since the 2001 anthrax attacks, which targeted Sen. Patrick Leahy, D-Vermont, and then-Majority Leader Tom Daschle, D-South Dakota.
On heightened alert
Suspicious letters in Michigan and Arizona, too
Investigators are trying to determine whether suspicious letters found at Senate offices elsewhere in the country came from the same source, federal law enforcement sources said.
Sen. Carl Levin, D-Michigan, said one of his home-state offices received a "suspicious-looking" letter and alerted authorities. "We do not know yet if the mail presented a threat," said Levin, the chairman of the Senate Armed Services Committee.
A staffer for Arizona Sen. Jeff Flake flagged "suspicious letters" at the freshman Republican's Phoenix office, Flake spokeswoman Genevieve Rozansky said in a statement, but "no dangerous material was detected in the letters."
Phoenix Fire Department spokesman Jonathan Jacobs said the envelope contained a powder. The person who found the envelope was being treated at a Phoenix-area hospital for a pre-existing condition and stress from the event, and others in the immediate vicinity were examined as well.
In a statement issued Wednesday, the FBI said it has no indication of a connection between the letters and Monday's bombings at the finish line of the Boston Marathon.
Ricin is easily made
Ricin is a toxic substance that can be produced easily and cheaply from castor beans. As little as 500 micrograms, an amount the size of the head of a pin, can kill an adult. There is no test for exposure and no antidote.
Experts say it is more effective for use against individuals than as a weapon of mass destruction.
Ricin was used in the 1978 assassination of Bulgarian dissident Georgi Markov. The author, who had defected nine years earlier, was jabbed by the tip of an umbrella while awaiting a bus in London and died four days later.
A ricin scare hit the Capitol in 2004, when tests identified it in a letter in a Senate mail room that served then-Majority Leader Bill Frist's office. The discovery led 16 employees to undergo decontamination; none was sickened, Frist said.