An envelope containing a white powdery substance was sent to one of White House hopeful Hillary Clinton's campaign offices in New York, though police initially ruled out any danger.

The envelope was first delivered to Clinton's offices in Manhattan, where campaign workers then transferred it to her Brooklyn headquarters, New York Police Department Lieutenant Thomas Antonetti told AFP.

"The preliminary investigation determined that it was negative in terms of containing a hazardous substance," he added, noting the Department of Health was conducting further evaluation to determine the nature of the substance.

Antonetti said the envelope also contained writing, but no death threats.

"We're trying to determine what the substance was. For right now, we can at least rule out any poisonous or deadly nature of the substance," Antonetti said.