The letter was addressed to Donald Trump Jr. and sent to his mother-in-law's apartment on East 54th Street, just east of First Avenue, in Manhattan.
This was the scene outside their NYC apartment early Monday afternoon:
There, Trump Jr.'s wife, Vanessa, opened the letter just after 10 a.m. She and two other people were decontaminated by firefighters at the scene. All three taken to the hospital for evaluation, FDNY officials said.
Everyone exposed to the powder appears to be fine, officials said.
"The substance was deemed to be non-hazardous and is being transported to a lab in New York City for further analysis," NYPD spokesman Peter Donald said.
PHOTOS: Scene of Donald Trump's apartment in NYC
Investigators said the letter was postmarked from Boston.
Police and Secret Service agents are at the scene investigating.