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 Sutton Place apartment early Monday afternoon:
There, Trump Jr.'s wife, Vanessa, opened the letter just after 10 a.m. Officials said Vanessa Trump began coughing after encountering the powder and immediately called 911, reporting that she felt nauseous.
FDNY firefighters decontaminated Vanessa Trump and two others. All three were taken to the hospital for evaluation and are expected 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, and they're now trying to determine who sent it.
Trump Jr. tweeted about the incident, saying he wishes he could be by his wife's side:
Trump Jr.'s sister, Ivanka Trump, posted a message of support for her family on social media:
And Vanessa Trump later posted a message of appreciation for the help from first responders:
Sources tell Eyewitness News the powder was corn starch. The identities of the two others take to the hospital have not been released.
Police and Secret Service agents are continuing to investigate.
Wife of Donald Trump Jr. taken to hospital after opening envelope containing white powder
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