Donald Trump Jnr's wife Vanessa was taken to a New York hospital yesterday after she opened a piece of mail containing an unidentified white powder that was later determined to be non-hazardous.
"The substance had arrived by mail and it was addressed to Donald Trump Jnr," said a New York Police Department spokesman.
Vanessa Trump, the daughter-in-law of US president Donald Trump, was taken to hospital after she complained of nausea following the exposure.
Three patients from the household were taken to the New York-Presbyterian/Weill Cornell Medical Centre for further evaluation, said a fire department spokeswoman.
The three included Vanessa Trump's mother, although she had not complained of symptoms.
The package had a Boston postmark, it was reported, but police officials declined to comment on that detail.
US authorities have been on alert for mail containing white powder since 2001, when envelopes laced with anthrax were sent to media outlets and US lawmakers, killing five people.
The US Secret Service, which is charged with protecting members of the president's family, has now joined the investigation.
President Trump spoke with his daughter-in-law yesterday afternoon, White House spokeswoman Sarah Sanders said, declining further comment on the investigation.
The apartment building where the incident happened is in Manhattan's Sutton Place neighbourhood close to the East River.
It may be the home of Vanessa Trump's mother, local news outlets reported.
Donald Trump Jnr, 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.
Congress has held investigations into those meetings and whether they were part of a Russian campaign to influence the 2016 US presidential election.
Russia denies trying to influence the election. President Trump has dismissed any talk of collusion.
In 2016, white powder, which also proved harmless, was sent to the home of Eric Trump, Donald Trump Jnr's brother.