The New York Police Department is looking for answers after finding half a dozen envelopes containing white powder mailed to city schools in the last two weeks.

The envelopes, each mailed in a “similar-looking white envelope,” were first received March 10, police said. The most recent envelope was found on Friday, one day after the NYPD first sent an alert about the suspicious mailings.

The NYPD said each of the envelopes was screened and found to be non-hazardous.

All six schools are located in Manhattan, though police did not specify which received the letters or whether they were public or private schools. The NYPD released an image of one of the envelopes, which had “middle school” written in all capital letters across the front and a U.S. flag stamp.

“It appears that the motive of the sender is to cause disruption and alarm,” the NYPD said in a statement.

Public schools have bounced back and forth between open and closed during the current year due to the COVID-19 pandemic. They were opened in the fall on a staggered approach, but then closed in November as cases rose. Schools for elementary and special needs students reopened in December and middle schools opened in February. High schools will reopen next week.

Mayor Bill de Blasio announced parents would be able to opt their students into in-person learning on Friday, with schools adopting new guidance from the Centers for Disease Control and Prevention.

The letters bring back memories of the anthrax attacks of 2001, which took place in the weeks after the attacks of Sept. 11. Multiple letters were mailed to media outlets in New York City, as well as some to legislative offices in Washington, and five people died. There have also been attacks with the poison ricin in the years since those anthrax mailings, including letters mailed to President Barack Obama and Mississippi Sen. Roger Wicker in April 2013.

A woman was arrested for mailing envelopes to the White House containing ricin in September 2020, though it did not reach its destination.

Those legitimate scares have sparked a litany of copy-cat mailings containing nontoxic material, such as the ones apparently mailed to the schools in New York City.

In March 2018, a man was arrested for sending envelopes containing a hoax white powder to Donald Trump Jr. and four others, including Michigan Sen. Debbie Stabenow. Trump’s then-wife, Vanessa, who opened the letter in New York City, was taken to the hospital as a precaution. Daniel Frisiello pleaded guilty and was sentenced to probation in the case.

Police asked anyone with information about the letters sent to NYC schools to call the NYC Terrorism Hot-Line at 888-NYC-SAFE. Anyone who finds a similar letter should call 911.

ABC News’ Aaron Katersky and Josh Margolin contributed to this report.