WASHINGTON (Reuters) - An envelope mailed to the White House tested "presumptive positive" for cyanide and will undergo more testing to confirm the results, the United States Secret Service said on Tuesday.

The agency charged with protecting the President said the envelope, which was received on Monday at the White House Mail Screening Facility, initially tested negative.

Biological testing on Tuesday "returned a presumptive positive for cyanide", the Secret Service said in a statement, adding the sample was then sent to another facility to confirm the results. The agency declined further comment, citing the ongoing investigation. There were no injuries from the envelope, according to media reports.

The Intercept news website, which first reported the incident, said the envelope contained a milky substance and was in a container wrapped in a plastic bag.