Democratic presidential nominee Hillary Clinton’s campaign headquarters received an envelope containing a white powdery substance Friday, causing workers to evacuate a floor of the Brooklyn building.

According to Lt. Thomas Antonetti of the New York Police Department (NYPD), the envelope was first delivered to Clinton’s midtown Manhattan office — just as all of her mail — and then transferred to the 11th-floor office at One Pierrepont Plaza in Brooklyn Heights, CNN reported.

The campaign workers opened the envelope, leading to the evacuation of the floor. The NYPD is in the process of testing the substance as a part of their investigation.

The Secret Service, NYPD’s Emergency Service Unit, Office of Emergency Management and the Department of Environmental Protection were all reportedly involved in the response.

Clinton was not in the city at the time of the discovery and evacuation. 

There has been a constant fear around toxic powders after powdered anthrax was mailed to a number of media outlets and members of congress as an aftermath of the Sept. 11 attacks in 2001. The poisonous substance led to the death of five people and infected 17 others, Daily Mail reported.

Clinton’s rival and Republican nominee Donald Trump has been at the receiving end of a number of “white powder” deliveries in the months before the elections, but all have been non-toxic.