Vanessa Trump, the wife of Donald Trump jnr, has been hospitalised after opening a letter containing white powder, according to senior police officials.
Vanessa Trump opened the letter at 10am in her home in Manhattan.
The president's daughter-in-law was taken to hospital after she complained of nausea following her exposure, along with two others as a precaution, New York officials said.
There's no indication they suffered any injuries and the identities of the other two people taken to the hospital weren't immediately clear.
"The substance was deemed to be non-hazardous and is being transported to a lab in New York City for further analysis," New York Police Department spokesman J. Peter Donald said.
Investigators are now looking into where the substance came from.
The area was decontaminated and police and other emergency units are still at the home.
Secret Service was also investigating, the NYPD 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 lawmakers, killing five people.