The cyanide was found in a letter containing a plastic package filled with a "milky substance," according to an internal law enforcement document. Reuters/Jonathan Ernst

The White House received a letter laced with cyanide poison, a Secret Service spokesman said on Tuesday. The package tested negative in initial testing for biological agents, but lab results received on Tuesday revealed a “presumptive positive” test for the poison, according to CNN. Now the package is being sent to another lab for confirmation.

Secret Service spokesman Brian Leary declined to comment further on the incident, as the investigation is ongoing. All packages sent to 1600 Pennsylvania Avenue are first processed at the White House mail screening facility outside Washington, D.C., in Maryland. Packages containing anthrax, cyanide, ricin and a number of other poisoned have been mailed to the White House.

The Intercept first broke the story on Tuesday when it obtained an internal document alerting staff to the positive test of the envelope, which contained “an unknown milky substance, in a container wrapped in a plastic bag.” The letter had the return address of an individual with a history of sending ill-intentioned letters dating back to 1995, including one that was covered in urine and feces, according to the Intercept.

A ricin-laced letter was intercepted at the screening facility in 2013 that bore a likeness to a ricin-laced letter that was sent to then-Mayor of New York City Michael Bloomberg, in which the sender threatened Bloomberg over his stance on gun control laws. That letter was given to the FBI Joint Terrorism Task Force for further investigation, according to the Los Angeles Times.

Cyanide stops cells from being able to use oxygen, causing them to die. The heart and brain are most heavily affected. Cyanide was weaponized in the early 20th century and used by Nazi Germany to kill millions of prisoners at its concentration camps. It can be made into a colorless gas or in crystal form, according to the Centers for Disease Control.