In response to that episode, all mail addressed to the White House and the Capitol is now screened at outside sorting facilities, where the letters to Mr. Obama and Mr. Wicker were intercepted. Although they showed evidence of ricin in a preliminary test, the F.B.I. cautioned that such tests are often faulty and said further testing was needed for a conclusive result.

“Only a full analysis performed at an accredited laboratory can determine the presence of a biological agent such as ricin,” the F.B.I. said in a statement. “Those tests are currently being conducted and generally take 24 to 48 hours.”

Leonard A. Cole, an expert in bioterrorism who teaches at the University of Medicine and Dentistry of New Jersey, urged caution. “We’re used to false positives,” he said. “I have no doubt that if there was no Boston Marathon bombing, we wouldn’t even be talking about the ricin letters right now.”

But Dr. Cole, who has written a book called “The Anthrax Letters,” said the parallels to that episode were unmistakable. People tend to seize on the publicity generated by terrorist attacks, he said, to perpetrate copycat attacks — some genuine, more of them hoaxes.

Ricin, if inhaled or ingested, can be every bit as lethal as anthrax. A byproduct of the castor oil plant, it is highly toxic. If converted into a powder, a few grains of it can be deadly, and there is no antidote. It was used most famously in 1978, when an assassin killed a Bulgarian dissident, Georgi Markov, on a London street by firing into his leg a tiny ricin-laced pellet from a modified umbrella.