The U.S. Secret Service says a letter addressed to the White House has tested positive for cyanide, a deadly poison.

The agency, which is charged with protecting the president and other high-ranking officials, issued a statement Tuesday saying the letter was received at a screening facility away from the White House campus the day before. The letter initially tested negative for biological agents, but the agency said further tests returned a “presumptive positive” for cyanide.

The letter has been sent to another facility for further testing. The Secret Service says it will have no further comment, due to the ongoing investigation.

According to a website called The Intercept, a return address on the cyanide-laced letter is a man who has sent multiple packages to the White House dating back to 1995, including one that was covered in urine and feces.

In 2013, letters addressed to President Barack Obama and Mississippi Senator Roger Wicker were laced with the poison ricin.  One person was arrested in the incident.

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