A letter addressed to US President Barack Obama was on Wednesday found to be carrying a granular substance that tested positive for poisonous Ricin.
The letter was intercepted at an off-site testing facility, and never made it to the White House.
A similar letter addressed to a senator was detected on Tuesday.
There have now been reports of more suspicious letters and packages being delivered to other lawmakers my a man carrying a backpack.
The Capitol hill police is investigating them.
The FBI, which is investigating these mails, said, “There is no indication of a connection to the attack in Boston.”
Ricin is a poison found naturally in castor beans. A dose as small as a few grains of table salt can kill an adult human. And it has no antidote, says the Centre for Disease Control and prevention.
Letters laced with another poisonous substance Anthrax were found to have been mailed to US lawmakers in the aftermath of September 11, 2002 attacks.