A preliminary field test on a white substance discovered in the White House has reportedly tested positive for cocaine, according to law enforcement officials, and the US Secret Service was investigating how it ended up at 1600 Pennsylvania Avenue on Tuesday.

The existence of the substance, which has been sent out for additional testing, was discovered late Monday when a firefighter from the Washington DC fire department's hazardous materials squad radioed: "We have a yellow bar saying cocaine hydrochloride," according to the Washington Post.

According to Secret Service spokesperson Anthony Guglielmi, the finding prompted an increased security alert and a brief evacuation of the executive residence when it was discovered during a regular examination.

Where was the substance found? 

Initial accounts revealed that the substance was discovered in a reference library. The Associated Press later reported on Tuesday, citing two sources who were not allowed to speak about the probe, that the white powder was discovered "in an area accessible to tour groups, not in any particular West Wing office."

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"An investigation into the cause and manner" of the substance's entry into the White House is underway. Joe Biden was at Camp David at the time, reported Guglielmi. On Tuesday morning, US President Barack Obama and First Lady Jill Biden returned to the White House from a presidential retreat in Maryland.

According to an official involved with the inquiry, the amount of the substance was minimal.

This is not a first

This would not be the first time that illegal substances made their way inside the White House. Snoop Dogg admitted to consuming marijuana in a restroom in 2013, and Willie Nelson admitted to smoking a joint on the White House roof during Jimmy Carter's presidency.

During a visit to the White House during first lady Nancy Reagan's "Just Say No" anti-drug campaign, British actor Erkan Mustafa admitted to smoking marijuana and doing a line of cocaine. Grace Slick of Jefferson Airplane said in 2011 that she attempted and failed to spike President Richard Nixon's tea with LSD in 1970.