An envelope addressed to the White House contained a "milky substance" that has tentatively tested positive for cyanide.
While an initial test Monday proved negative, more biological testing Tuesday had a "presumptive positive" result, The Associated Press reports.
The letter, which was received Sunday, is set to be tested again at another facility to confirm the positive result, according to a statement from the Secret Service, which declined further comment as the investigation is ongoing.
The contaminated letter did not cause any injuries, according to CBS.
The Intercept, which first reported about the envelope, obtained an internal law enforcement document that states the letter including the "milky substance" was in a container wrapped in a plastic bag. The return address is linked to a man known by the Secret Service to have sent a urine and feces-covered package in the past, according to The Intercept.