Further tests are being conducted on an envelope posted to the White House which may contain cyanide, the Secret Service says.
Tests at a post screening facility were found to have traces of the poison.
The Secret Service, which is responsible for President Barack Obama’s security, said an investigation was under way.
Initial tests on March 16 were negative but more tests on March 17 returned a “presumptive positive” for cyanide.
“The sample was transported to another facility to confirm the results,” Secret Service spokesman Robert Hoback said in a statement.
He added there would be no further comment.
There were no injuries or exposure concerns for the screening facility employee who first opened the letter, a law enforcement official told CNN.
The discovery was first disclosed by The Intercept, which obtained an internal law enforcement document.
On March 17, the Secret Service director Joseph Clancy was questioned by a congressional committee about a series of recent security lapses.
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