Police are investigating a series of incidents in which six schools in Manhattan anonymously received an envelope containing a white powdery substance.

Police shared a picture of one of the envelopes that contained the white dust on social media, asking anyone with relevant information about the incidents to come forward.

"In each instance, the material was screened and determined to be non-hazardous," the NYPD said in a statement.

The incidents first began on March 10th, when an envelope was reportedly mailed to the Sen. Robert F. Wagner Middle School on the Upper East Side. This was later followed by Montclare Children's School on the Upper West Side a day later. On March 18th, the Success Academy High School of the Liberal Arts in Midtown and NY Kids Club and New York Preschool in the Upper West Side were also forwarded an envelope. The River Park Nursery School in the Upper West Side was sent two envelopes, one on March 16th and the other on March 19th.

In each instance, the envelopes had a U.S. flag stamp placed and a "handwritten address in block letter[s]," according to police. Investigators are trying to determine why these schools were specifically targeted.

A similar incident happened a month ago in several schools in Georgia, in which anthrax was actually detected. The FBI is now investigating the matter.

It’s unclear how many students and staff were present in these school buildings when the letters were received.

Those who may have information should reach out to the NYC Terrorism Hotline at 888-NYC-SAFE.