The letter was intercepted at a screening facility outside the White House, the Secret Service said.
It was received Tuesday - similar timing as the letter addressed to Sen. Roger Wicker, R-Miss., that tested positive for ricin. The letter addressed to the president had similar markings and was similar in appearance to the one addressed to Wicker, according to a law enforcement official.
In a statement, the FBI said the letter contained "a granular substance that preliminarily tested positive for ricin." The bureau also said that ricin had been discovered at another mail screening facility.
The mailing facility is not close to the White House grounds, the Secret Service said. An official said the Secret Service was working with the Capitol Police and the FBI.
On Capitol Hill, the Hart Senate Office Building was shut down Wednesday, and no one was allowed to enter, but the building was not evacuated. Capitol Police officers were yelling at staff members in the hallways to get back in their offices.
"Apparently there was a package over there, and they said to walk the way I have," Sen. Ron Wyden, D-Ore., said as he left the Hart building heading to the Dirksen Senate Office Building.
The Capitol Police confirmed that a suspicious package was found on the atrium level of the Hart building, as well as on the third floor of the Russell Senate Office Building.
They also said that officers were talking to a man in the Hart building about the suspicious packages.
Sen. Carl Levin, D-Mich., also released a statement Wednesday saying that a suspicious letter had been found at his office in Saginaw, Mich. After the letter found to contain ricin was sent to Wicker, the senators were briefed on protocol for their state offices to follow if they received a suspicious letter.
"Earlier today, a staffer at my Saginaw regional office received a suspicious-looking letter," said the statement released by Levin's office. "The letter was not opened, and the staffer followed the proper protocols for the situation, including alerting the authorities, who are now investigating. We do not know yet if the mail presented a threat. I'm grateful for my staff's quick response and for government personnel at all levels who are responding."