Aside from suicides and accidental poisonings, the only known killing by ricin was Georgi Markov, a Bulgarian journalist who was stabbed by an umbrella on a London street in 1978. The umbrella's tip injected a tiny metal capsule containing ricin into Markov's leg. He died three days later.
In lab tests involving monkeys, powdered ricin has been found to cause bleeding in the lungs and suffocation within days.
At a White House briefing, spokesman Jay Carney said Obama had been briefed but that he did not "have a way to characterize" the president's reaction.
"Obviously he understands and we all understand that there are procedures in place," Carney said. "There's a process in place that ensures that materials that are suspicious or substances that are found to be suspicious at remote locations are then sent for secondary and more intense testing, and that process is underway now."
On the question of whether there might be any link with the bombings in Boston, Carney deferred to the FBI, which said Wednesday it did not immediately see any connections between the cases.
The letter to Obama was intercepted Tuesday at a Secret Service-run mail sorting facility in Washington's Anacostia neighborhood, according to law enforcement officials.
The FBI said in a statement Wednesday that the letter contained "a granular substance that preliminarily tested positive for ricin." Authorities did not say when or where the letter was postmarked.
The other letter, to Wicker, was uncovered Tuesday at a Prince George's County, Md. sorting facility where congressional mail has been screened since anthrax-laced letters were sent to Capitol Hill in 2001. The letter to Wicker was postmarked from Memphis and contained no return address and no "outwardly suspicious" markings, Senate Sergeant at Arms Terrance Gainer wrote to senators in an e-mail.