An envelope addressed to the White House has tentatively tested positive for cyanide, the US Secret Service says.
An off-site mail screening facility reported the letter on Monday. Initial biological testing was negative.
The Secret Service, responsible for President Barack Obama's security, says additional testing on Tuesday returned a "presumptive positive" for cyanide.
A sample has been taken to another facility for further testing while an investigation into the letter continues.
The Secret Service says it won't be commenting further on the matter.
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