Vanessa Trump called 911 Monday after opening the letter, addressed to her husband, at their Manhattan apartment.
Firefighters decontaminated her and two others at the scene, and she was taken to a hospital afterward.
"You're an very bad person and now you get what you deserve", the four-sentence note addressed to Donald Trump Jr read, according to NY media. "Eric looks smart", the letter read, according to officials familiar with the case. This is why people hate you. "So shut the f-- up".
Vanessa Trump, the wife of US President Donald Trump's eldest son, has been taken to hospital after police say she opened an envelope sent to her home which contained white powder.
Junior stated it was "truly disgusting" that some individuals would choose to express political dissident and opposing views in such a disturbing manner.
The Secret Service, in a statement, confirmed it's conducting an "active investigation", but declined to reveal details.
President Donald Trump's personal lawyer, Michael Cohen, said in response to the incident. "This unsafe and reckless act goes beyond political differences".
Vanessa Trump, 40, wasn't hospitalized but was tested at the hospital out of an abundance of caution.
"Thinking of [Vanessa Trump] & wishing I was by her side today", Ivanka Trump wrote.
Sending threatening letters (with or without corn starch) is never okay.