Detective Mindy Diaz says no injuries were reported after police were called to the building in lower Manhattan on Thursday afternoon.
All together, seven envelopes containing the powder were found in various locations downtown, including American Express and JPMorgan Chase, police said.
Diaz says three people were exposed to the powder, and Detective Cheryl Crispin noted that those individuals have been decontaminated. Authorities say there's no indication that the powder poses a threat, but testing is ongoing to make sure.
Police say the envelopes were sent from Maryland.
American Express spokeswoman Joanna G. Lambert says business has not been interrupted.
In October and November 2001, five people were killed after coming in contact with mailings containing anthrax.
In November, at least four U.N. foreign missions received packages containing suspicious powder. The most recent hoaxes involved letters that were sent from Texas.
Authorities say it's too soon to know if there is any possible connection between who sent the earlier envelopes and the ones received this week.