Trump's wife, Vanessa, opened the letter and was taken to a hospital as a result, according to the New York Police Department.
She was "not feeling well" after opening the letter, police said. The Associated Press reported that she was coughing and feeling nauseated.
In a preliminary assessment, police believe the substance is not suspicious, said Lt. John Grimpel, a department spokesman.
Trump Jr., along with his brother, Eric, took over managing the Trump Organization after their father won the presidency. The company did not immediately respond to a request for comment Monday.
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