Two days after the Boston bombings, a letter addressed to President Obama has tested positive for the deadly poison ricin.
A similar ricin-tainted letter had been sent to the Republican senator from Mississippi, Roger Wicker.
Capitol Hill police were also searching other suspicious letters Wednesday.
White House spokesman Jay Carney said, "For a long time now, there have been long established procedures and protocols. Any time a suspicious powder is located in a mail facility, it is tested."
At least one of the letters was postmarked Tennessee, raising the specter of domestic terrorism. Now, the question everyone is asking is: Could there be a connection between the ricin letters and the Boston Marathon bombings?
Harvey Kushner, author of The Encyclopedia of Terrorism, said the marathon bombings bear the hallmarks of home-grown terrorists, not Al-Qaeda.
"The fact that there is no credit being taken by a foreign group indicates to me that it is some wannabe lone-wolf type of domestic terrorism who perpetrated this act of terror" he said.
Former Secretary of State Condoleezza Rice is also struck by the fact that no one has claimed responsibility for the Boston attack.
"I do think that's odd because normally these jihadist groups, the biggest ones like Al-Qaeda, would take some kind of responsibility," she said.