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The US Secret Service says it is conducting further tests on an envelope posted to the White House which may contain cyanide.

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 Monday were negative but more tests on Tuesday 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.

Earlier on Tuesday, the Secret Service director Joseph Clancy was questioned by a congressional committee about a series of recent security lapses.