The suspicious substance was found at a Manhattan campaign office
Hillary Clinton's campaign headquarters has been evacuated after a white powdery substance was discovered in an envelope.
New York Police said four people were exposed to the substance but no injuries or illnesses have been reported.
The suspicious substance was found by two interns at a Manhattan campaign office and then taken to Mrs Clinton's Brooklyn headquarters.
It is currently being tested.
The 11th floor of the building was evacuated.
It was not immediately known what the substance was or whether it was harmful and police are still investigating.
On November 8th, Democrat Mrs Clinton takes on Republican Donald Trump in the US presidential election.
She currently leads in national polls after her rival's campaign was rocked by a series of sexual misconduct allegations against the billionaire businessman.