The FBI confirmed on Wednesday a letter addressed to President Barack Obama has been preliminarily tested positive for ricin but the authorities have not found a link to Monday's Boston bombings.

"The envelope, addressed to the President, was immediately quarantined by US Secret Service personnel, and a coordinated investigation with the FBI was initiated," said the FBI in a statement.

The FBI said the suspicious letter addressed to Obama and filters at a government mail screening facility were both preliminarily tested positive for ricin on Wednesday. Mail from that facility is also being tested.

"Only a full analysis performed at an accredited laboratory can determine the presence of a biological agent such as ricin. Those tests are currently being conducted and generally take 24 to 48 hours," the FBI said.

A US Secret Service spokesman said on Wednesday that a letter addressed to Obama contained "suspicious substance."

The spokesman said that the authorities intercepted the suspicious letter at the White House off-site mail facility. The letter arrived on Tuesday, the same day that another letter addressed to a US senator was intercepted and initially tested positive for ricin, a highly toxic substance.

Two Senate buildings were evacuated on Wednesday morning due to reports of suspicious mails.

While the investigation into the letters remains ongoing, the FBI also said in the statement that there was no indication of a connection to the attack in Boston.

Lawmakers on the Capitol Hill confirmed Tuesday that the Capitol Police intercepted a suspicious letter, which was addressed to Senator Roger Wicker, at an off-site postal screening center on Tuesday morning.

The incidents came one day after two explosions occurred near the finish line of the Boston Marathon, killing three people and wounding over 170 others. The authorities have called the Boston bombings an "act of terror," but the motives and perpetrators remain unknown.