The substance was first discovered by two interns at a campaign office in Manhattan.
The campaign headquarters for US Democratic presidential candidate Hillary Clinton was evacuated on Friday night after a white substance was discovered in an envelope, police said.
The substance was first discovered by two interns at a campaign office in Manhattan and was taken to Clinton's Brooklyn headquarters, triggering the evacuation of the 11th floor, a New York Police Department spokesman said.
A total of four people were exposed to the substance, police said, though there have been no injuries or reported illnesses. Police are testing the substance and investigating the case.
Further details were not immediately available.
© Thomson Reuters 2016