An envelope mailed to the White House tested positive for cyanide, the Secret Service said.
The envelope arrived on Monday at a facility elsewhere in the Washington area that screens mail for the White House. Biological tests were negative, but a chemical test on Tuesday came back positive for cyanide, the agency said.
The envelope was sent to another facility to confirm the result.
According to the website The Intercept, a law enforcement alert said that the return address was for a man who has a Secret Service record dating to 1995, including sending a package covered in urine and feces.
The alert said that the envelope contained an "unknown milky substance" in a container wrapped in a plastic bag, The Intercept reported. The Secret Service had no further comment.
People exposed to cyanide in larger amounts can suffer convulsions, lose consciousness and die of respiratory failure, according to the Centers for Disease Control and Prevention.