FILE - In this Jan. 20, 2017 file photo, Vanessa Trump arrives at the U.S. Capitol on Friday, Jan. 20, 2017, in Washington, Donald Trump's inauguration ceremony. Police say Vanessa Trump, wife of Donald Trump Jr., opened an envelope that contained white powder, felt ill and was taken to New York City hospital as a precaution. A police department spokesman says a preliminary test of the powder indicated it wasn't dangerous. (Win McNamee/Pool Photo via AP, File)

FILE - In this Jan. 20, 2017 file photo, Vanessa Trump arrives at the U.S. Capitol on Friday, Jan. 20, 2017, in Washington, Donald Trump's inauguration ceremony. Police say Vanessa Trump, wife of Donald Trump Jr., opened an envelope that contained white powder, felt ill and was taken to New York City hospital as a precaution. A police department spokesman says a preliminary test of the powder indicated it wasn't dangerous. (Win McNamee/Pool Photo via AP, File)

NEW YORK (AP) — 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, though police later said the substance wasn’t dangerous, authorities said.

The frightening episode happened after 10 a.m. when 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 said.

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“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 couldn’t 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.

White House spokeswoman Sarah Huckabee Sanders said 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.

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 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 organizations and the offices of two U.S. senators. Those letters killed five people.

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This story has been corrected to show the envelope was sent to Vanessa Trump’s mother’s apartment, not hers.