A person familiar with the matter said that results on the substance were inconclusive, but officials do not believe it was deadly. The person spoke on condition of anonymity to confirm details of the letter, which were first reported by The Daily Beast.

Trump Jr. opened the letter, which also contained a death threat, in his home office, and emergency responders wearing hazmat suits responded.

Jupiter police said the investigation is being handled by the Palm Beach Sheriff's Office, which said it was working with the Secret Service but didn't have any further details. 

Trump Jr. is one of his father's top campaign surrogates, frequently headlining events and appearing in interviews on his behalf. 

It's the second time white powder has been sent to the former president's oldest son. In 2018, his then-wife, Vanessa, was taken to a New York City hospital after she opened an envelope addressed to her husband that contained an unidentified white powder. Police later said the substance wasn't dangerous.

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 twice in 2016 to Trump Tower, which served as Trump's campaign headquarters.

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.