Donald Trump Jr.’s wife and mother-in-law were hospitalized as a precaution on Monday morning after opening an envelope containing a white powdery substance that investigators later determined was cornstarch and was not hazardous, police officials said.

The letter, addressed to Donald Trump Jr., was sent to the Manhattan apartment of his mother-in-law, Bonnie Haydon, the officials said.

Ms. Haydon passed the letter to her daughter, Vanessa Trump, around 10 a.m. When she opened it, at least one of them appeared to start coughing and felt nauseous, one of the officials said.

They called 911, and the New York Police Department and Fire Department responded to the apartment, on East 54th Street in the Sutton Place neighborhood.