US Capitol Police say they have re-opened the Hart Senate building, following an evacuation.
Tests on a suspicious package were negative, Reuters reports.
Michigan Senator Carl Levin says his regional office has received a suspicious letter, reports Associated Press.
White House Press Secretary Jay Carney has briefed the media, following two suspicious letters addresSed to the President and a US Senator.
He said: "There was a letter addressed to the President at an off-site mail facility. It was noticed to have contained a suspicious substance and tests were undertaken.
"The FBI has the lead in that investigation and has said they will be conducting further tests to determine what the nature of the substance is.
"There was another letter detected by Capitol Police that was sent to a US Senator that is also part of the investigation by the FBI...The President, of course, has been briefed on these letters."
US Capitol Police says they have cleared parts of two Senate office buildings over suspicious letters, reports Reuters.
Police say they are questioning an individual over "suspicious envelopes". Two letters - one addressed to Barack Obama and the other addressed to Senator Roger Wicker - have been found to contain the poison ricin.
A letter addressed to President Barack Obama containing a suspicious substance was received at a White House mail screening facility on Tuesday, the US Secret Service has said.
Fox News reporter Chad Pergram has tweeted that the letter sent to Barack Obama and the letter sent to Senator Roger Wicker yesterday - which tested positive for the poison ricin - contained 'similar language' and were 'signed identically'.
A suspicious letter sent to US President Barack Obama has been intercepted at a mail screening facility, Reuters reports.
The letter addresses to Obama, contained a "suspicious substance" authorities have told NBC News.
"A letter addressed to the president was received at a remote screening facility containing a suspicious substance. We are working closely with Capitol Police and the FBI in this investigation," a Secret Service spokesman said.
It comes as it was reported that two letters containing the poison ricin were sent to the US Senate yesterday and today.
Terry Moran, of ABC News, has tweeted that a second letter sent to the US Senate has tested positive for the poison ricin.
The US Senate sergeant at arms said in a warning to members of the Senate that all employees should be vigilant in the wake of a poisoned letter sent to a Senator.
Terrance Gaines said in a written statement: "Senate employees should be vigilant in their mail handling processes for ALL mailings."
Members of the Senate were briefed on the ricin incident by Mr Gaines during a meeting with FBI director Robert Mueller and Janet Napolitano, the secretary of Homeland Security, on Tuesday on the bombings in Boston.
The two incidents have not been connected.
Several senators told reporters after the briefing that the incident reminded them of the anthrax attacks in the wake of the 9/11 attacks on the Pentagon and World Trade Center.
Several senators said that all mail to the US Senate had been stopped and post offices at the Capitol had been closed as a precaution.
They said they were getting in touch with their state offices, where mail is not routinely screened, to ensure that precautions were being put in place.