Vanessa Trump, the wife of Donald Trump Jr, and two other people have been taken to a hospital in New York after she opened a piece of mail containing an unidentified white powder.
The substance was later determined to be non-hazardous, officials said.
"The substance had arrived by mail and it was addressed to Donald Trump Jr," New York Police Department spokesman Carlos Nieves said.
Ms Trump, the daughter-in-law of US President Donald Trump, was hospitalised after she complained of nausea following her exposure, city officials said.
Three patients from the household were transported to the New York-Presbyterian/Weill Cornell Medical Centre for further evaluation, Fire Department spokeswoman Sophia Kim said.
The three also included Ms Trump's mother, though she had not complained of symptoms, the police spokesman said.
US authorities have been on alert for mail with white powder in it since 2001, when envelopes laced with anthrax were sent to media outlets and US politicians, killing five people.
The US Secret Service, which is charged with protecting members of the president's family, has joined the investigation, said spokesman Jeffrey Adams.
White House officials had no immediate comment.
The apartment building where the incident occurred is in Manhattan's expensive Sutton Place neighborhood close to the East River.
The apartment may be the home of Ms Trump's mother, local news outlets reported.
Mr Trump Jr, the president's eldest son, has been in the public eye for his role in 2016 meetings with a Russian attorney and others where the Trump campaign was offered potentially damaging information about Democratic rival Hillary Clinton.
