An envelope sent to the White House mail facility tested positive for cyanide, a media report said on Wednesday.
"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," an internal law enforcement document said.
According to the document, the envelope listed a return address for a man who has a record with the Secret Service dating back to 1995, The Intercept news reported.
The individual has sent multiple packages over the years, the most recent was received on June 12, 2012, and contained mini alcoholic beverages, the document added.
The initial biological examinations conducted on Monday tested negative.
On Tuesday, however, the chemical testing returned positive for cyanide.
The sample has been send to another facility to confirm the results.