US President Barack Obama has also been targeted with a poisoned letter as bomb and bio scares spread across Washington after Monday’s bombings in Boston, echoing 9/11 and anthrax attacks in the country in 2001 that killed thousands.

Although letters and packages sent to the US president and members of Congress are regularly checked for suspicious substances at two secret postal locations near Washington, the heightened state of alert prompted by the bombings this week in Boston has reportedly led to the ' positive identification ' of two letters, one addressed to Obama and the other to Republican Mississippi Senator Roger Wicker, for containing the lethal toxin ricin, according to major US media outlets. At least five other senators were reportedly forced into an emergency mode for receiving “suspicious” packages at their offices in Washington or their home states, “prompting evacuations of their staff and lock downs of many more, ” according to a “Washington Post” report on Thursday. Furthermore, US police ordered thousand of congressional staffers and aides not to leave their offices after a bag was reportedly sighted at the entranceway of a Senate office building, as a bomb squad raced towards the Capitol Hill. Two hours later, however, the package, as well as two letters delivered to the officers of two senators, “was cleared, ” the report adds.

“The ricin scare, ”The Postcontends, “had spilled into public view in dramatic fashion early Wednesday, less than two days after the deadly bombings at the Boston Marathon and bringing with it an eerie echo of the waves of fear that followed the terrorist attacks of Sept. 11,2001. ”

Meanwhile, the FBI, a domestic US intelligence and investigative agency, has announced the arrested of a Mississippi man, identified as Paul Kevin Curtis, as the suspect in the poisoned letter mailings, adding that he has also sent a third ricin - contaminated letter to “a Mississippi justice official. ” Although US authorities claim that Curtis is “well known to law enforcement as a frequent letter - writer to lawmakers, ” American media reports and law - enforcement officials have not elaborated on his background and his issues or concerns. The letters sent to Obama and Wicker were reportedly similar in content and the origin of their postmark, Memphis, Tennessee. They read, " To see a wrong and not expose it, is to become a silent partner to its continuance. " Both of them were signed, " I am KC and I approve this message. " Despite all the official frenzy, however, US media reports clearly indicate, citing official sources, that positive testing of the letters sent to Obama and Wicker are not yet definitive. Late Wednesday, according to thePostreport, FBI stated that is was still waiting for a “final word on whether the letters to Obama and Wicker definitely contained ricin. ”

“The initial tests can be inaccurate, ” the report underlines, adding that in 2004 a letter sent to a top US senator was initially believed to contain ricin but additional tests proved it was harmless.

Moreover, the report cites another law enforcement official as saying that more testing to verify the presence the poisonous agent, “using fluorescent antibodies, chromatography and searching for DNA, ” can take 24 to 48 hours. The developments come as no suspects have been arrested or identified for the Boston Marathon bombings. News reports, however, cite US authorities as saying they are looking for one or two individuals sighted in surveillance videos.