WASHINGTON — An envelope addressed to the White House contained a substance that federal investigators identified as the lethal substance ricin, according to a law enforcement official briefed on the inquiry.

It is unclear when the envelope was intercepted before it reached the White House mail room, but investigators believe that it was sent from Canada, according to the official.

Federal investigators are working to track down who sent it and determine whether other envelopes have been sent through the postal system.

Ricin, which is part of the waste produced when castor oil is made, has no known antidote.

The White House could not be immediately reached for comment.

In 2011, four Georgia men were arrested and later sentenced to prison for plotting to spread the toxin simultaneously in five U.S. cities, targeting federal and state officials. That same year, U.S. counterterrorism officials said they were increasingly tracking the possibility that al-Qaida would use ricin in attacks against the United States.

Two years later, a Mississippi man sent letters containing ricin to President Barack Obama and a Republican senator in an attempt to frame a rival. The letters were intercepted at sorting facilities.

In 2014, Shannon Richardson, an actress, was sentenced to 18 years in federal prison for mailing letters laced with ricin in May 2013 to multiple people, including Obama and Michael Bloomberg, the mayor of New York at the time.