The U.S. Secret Service has found out after two rounds of testing that a letter sent to the White House contains traces of the chemical cyanide.

In an internal memo obtained by The Intercept, Secret Service officials said on Monday that initial biological testing for the letter tested negative, but a second chemical analysis returned a "presumptive positive" on Tuesday. Officials say the letter has been sent to another unspecified facility for further testing.

"An envelope containing an unknown milky substance, in a container wrapped in a plastic bag, received at the White House Mail Screening Facility, tested positive for Cyanide," the memo says.

Secret Service officials say the letter is marked with a return address associated with a man who is known to have been sending letters of questionable content to the White House since 1995, including a letter covered in urine and feces. He sent the most recent package on June 12, 2012, a letter that contained miniature bottles of alcoholic beverage.

It is not known to whom the cyanide-laced letter was addressed. Secret Service officials received the letter on Sunday, March 15, in a Washington D.C. testing center located outside of the White House grounds away from the executive quarters. The facility was put up following a series of deaths and poisonings resulting from letters containing anthrax bacteria sent to White House officials, members of Congress, and journalists after the Sept. 11 attacks.

This is not the first time the Secret Service, which is responsible for the security of the President and his immediately family, has intercepted letters of a suspicious nature mailed to the White House.

In January 2014, James Everett Dutschke of Tupelo, Mississippi was sentenced to 25 years in prison after being found guilty of sending letters laced with the poison ricin to Obama, Republican Sen. Roger Wicker of Mississippi, and Mississippi judge Sadie Holland. The letters sent to the White House and Congress were both intercepted by the Secret Service, but the letter addressed to Holland reached the judge, although she was unharmed.

Another letter sent in 2013 was found to contain death threats, laced with profanity and racial slurs, against the president and his immediate family. In June that year, 20-year-old Ryan Kirker of McMechen, West Virginia was indicted on charges of threatening to kill the president but was released after forensic analysis conducted by the Secret Service showed Kirker's handwriting did not match with the handwriting on the letter.

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