Police evacuated the 11th floor of Hillary Clinton’s Brooklyn campaign headquarters Friday night after a white powder was found in a letter, an NYPD spokesman said.

The Hillary for America campaign notified law enforcement authorities Friday evening "of an envelope containing a suspicious substance," Glen Caplin, Hillary for America senior national representative, said in an email statement.

Preliminary analysis has found that the substance was not harmful, police said on Saturday, and that it is undergoing further testing. The initial testing was done "by federal and local officials," Caplin said.

Four people were exposed to the letter, but none has shown signs of illness. 

"The four individuals involved have reported no health issues and, following a full examination by medical personnel, were each released to go home," Caplin said.

The substance arrived through the mail at Clinton's campaign office in Manhattan, where a letter was opened by two people, and was taken to the headquarters in Brooklyn Heights, police said. Two additional people handled the letter in the Brooklyn office at One Pierrepont Plaza, the NYPD said.

The floor was evacuated as a precaution, though the office did not close.

"Our office remained open throughout this period and will remain open without interruption," Caplin said.

The NYPD Emergency Services Unit, Office of Emergency Management and the Secret Service were at the scene Friday night.

An NYPD spokesman said police were still investigating the precise nature of the substance, as well as the identity of the sender.

With Reuters