An envelope has been sent to the White House that may contain cyanide, the US Secret Service has confirmed.
Initial biological testing did not spot any traces of the poisonous substance but additional testing returned a "presumptive positive" for cyanide, according to Secret Service spokesman Robert Hoback.
The envelope, which was received on 15 March, is being sent to another facility for further testing. The Secret Service said its investigation into the incident is continuing.
An official told CNN there were no exposure concerns for the screening facility employee who handled the letter.
The envelope bore a return address on it for a man who has allegedly sent packages covered in urine and faeces in the past, according to The Intercept.
It said the most recent package sent from that address was received by the White House in June 2012 and contained mini alcoholic drinks.
In 2013, letters sent to Mr Obama, Senator Roger Wicker, and Mississippi judge Sadie Holland tested positive for the poison ricin.
The letters addressed to the president and to the senator were intercepted before delivery, but one letter reached Ms Holland. She was unharmed.
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