NEW YORK (CBSNewYork) — Three people were decontaminated at Trump Tower Thursday night after a suspicious white powder was found in a letter to the presidential frontrunner.

Authorities told 1010 WINS the substance turned out to be non-hazardous and harmless.

The white powder was found in the fifth floor office at Trump Tower, at 725 Fifth Ave. in Midtown around 8 p.m., the FDNY told CBS2.

The office is used by the Donald Trump presidential campaign, and is used by volunteers to make calls and open mail, sources told CBS2.

Sources said a young male intern opened a piece of mail addressed to Donald Trump, and the white powder spilled out, CBS2’s Tracee Carrasco reported.

Security at the building resealed the envelope and called 911, sources said.

The intern was decontaminated as a precaution, along with two other people, sources said. In all, five civilians and a police officer were exposed, the FDNY said.

The New York City Department of Health and Mental Hygiene and the NYPD Emergency Services Unit were called to the scene, sources said. The Secret Service and the NYPD were investigating.

Police Thursday night were testing the substance, but there was no indication that it was dangerous, sources said. But even if the substance turns out to be harmless, police said whoever sent the mail will be prosecuted.

Just last month, police and the FBI investigated a suspicious piece of mail that was sent to Donald Trump’s son, Eric Trump. Investigators said the letter, postmarked from Massachusetts, was opened by Eric Trump’s wife, Lara, at the Trump Parc East building on Central Park South on the night of Thursday, March 17.

Sources told CBS News that a white powder was also found inside of that envelope.

Sources said the March 17 letter warned that if Donald Trump does not drop out of the presidential race, his children could be harmed.

Donald Trump was holding a rally in Costa Mesa, California Thursday night.