The envelope was immediately quarantined by U.S. Secret Service personnel.
Ricin is a highly toxic poison—as little as a few grains can kill adult humans. Ricin is typically found in castor oil, coming from castor beans.
At a second government mail screening facility, Ricin was found in the filters. Mail is being tested at that facility, as well.
President Obama has been briefed about the situation, said Jay Carney, White House press secretary.
The FBI is sending the envelope to a lab and expects results in 24 to 48 hours. There is no indication of a connection to the bombings at the Boston Marathon.
There is a connection with another letter recently mailed to senator Roger Wicker, a Mississippi Republican, according to a law enforcement source.
Michigan Sen. Carl Levin is saying his office received a suspicious letter on April 17.
Obama will speak in Boston tomorrow at an interfaith service.
The Associated Press contributed to this report.