The US Secret Service, which is charged with protecting members of the president's family, has joined the investigation, said spokesman Jeffrey Adams. You are getting what you deserve.
"You're an terrible person and now you get what you deserve", the four-sentence note addressed to Donald Trump Jr read, according to NY media.
Vanessa Trump opened the letter and found an unknown substance inside.
Donald Trump Jr's wife opened the letter on Monday and was hospitalised.
Vanessa Trump was taken to a hospital for observation, also strictly as a precaution, according to three senior officials.
The New York City Police Department (NYPD) said initial tests did not indicate the powder was harmful but they are continuing to test the substance. She threw the letter into a plastic bag and called 911. The white powder was later discovered to be non-hazardous, and no one was harmed.
The powder was later determined not to be hazardous, NY officials said.
USA authorities have been on alert for mail containing white powder since 2001, when envelopes laced with anthrax were sent to media outlets and U.S. lawmakers, killing five people. "Truly disgusting that certain individuals choose to express their opposing views with such disturbing behavior", he tweeted, describing the matter as "incredibly scary".
As you may know, Vanessa and Trump got married back in November of 2005.
Mrs Trump's five children were not home at the time. Envelopes containing white powder were also sent to Trump Tower, twice in 2016.
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