According to cnn.com, a letter sent to the president has been found to contain a "suspicious substance".

Previously, another letter was found in the Senate mailroom, which is suspected of containing the poison, ricin.

"A letter addressed to the president containing a suspicious substance was received at the remote White House mail screening facility," Secret Service spokesman Brian Leary said. The Secret Service, FBI and Capitol Police are investigating, he said.

According to abc news, the letter to Obama has field tested positive for ricin, but it takes 24 to 48 hours to confirm with a full analysis at a lab, currently underway.

Authorities say there is no indication of a connection to the attacks in Boston.