Donald Trump Jr's wife was taken to a New York City hospital on Monday as a precaution after she opened an envelope addressed to her husband that contained an unidentified white powder.
The frightening episode happened when Mrs Trump, 40, opened the letter addressed to the president's son at her mother's midtown Manhattan apartment, investigators said.
She called 911 and said she was coughing and felt nauseous. Police later said the substance was not dangerous.
"Thankful that Vanessa & my children are safe and unharmed after the incredibly scary situation that occurred this morning," Donald Trump Jr. wrote on Twitter. "Truly disgusting that certain individuals choose to express their opposing views with such disturbing behavior."
The New York Fire Department said it treated three patients who were then taken to a hospital for evaluation.
Police said the envelope contained a letter but provided no other details.
Detectives from the New York Police Department's intelligence division and Secret Service agents were investigating.
Secret Service Special Agent Jeffrey Adams said in a statement that agents were investigating "a suspicious package addressed to one of our protectees" in New York but said he could not comment further.
Vanessa Trump, a former model, and Donald Trump Jr have five children, none of whom were home at the time of the incident.
Thankful that Vanessa & my children are safe and unharmed after the incredibly scary situation that occurred this morning. Truly disgusting that certain individuals choose to express their opposing views with such disturbing behavior.— Donald Trump Jr. (@DonaldJTrumpJr) February 12, 2018
White House spokeswoman Sarah Huckabee Sanders said on Monday that the president spoke by phone to Vanessa Trump about the incident.
Later, the Republican president's daughter-in-law posted a tweet thanking New York's police and fire departments, the FBI and the Secret Service for their help.
"I appreciate all the quick response to make sure that I was safe! Thank you," she wrote.
The Trump family has had to deal with a number of similar scares.
A special thanks today to the great men & women of the @NYPDnews @FDNY @SecretService @NewYorkFBI and the Joint Terrorism Task Force for their decisive action and incredible words of support to my wife and family. Their professionalism under pressure made a rough day manageable.— Donald Trump Jr. (@DonaldJTrumpJr) February 12, 2018
In March 2016, police detectives and FBI agents investigated a threatening letter sent to the Manhattan apartment of Donald Trump Jr's brother, Eric, that also contained a white powder that turned out to be harmless. Envelopes containing white powder were also sent to Trump Tower, which served as Mr Trump's campaign headquarters, twice in 2016.
Hoax attacks using white powder play on fears that date to 2001, when letters containing deadly anthrax were mailed to news organisations and the offices of two US senators. Those letters killed five people.