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An envelope with suspicious powder sent to one of President Donald Trump's sons also included a threatening note, senior law enforcement officials told NBC News.

“You are an awful, awful person. I am surprised that your father lets you speak on TV," the note to Donald Trump Jr. said, according to the officials.

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It also said: "You the family idiot. Eric looks smart. This is the reason why people hate you. You are getting what you deserve. So shut the f*** up.”

Related: Vanessa Trump taken to hospital as precaution after white powder sent to her home

Trump Jr.'s wife, Vanessa Trump, and two other people were taken to a New York hospital Monday as a precaution.

New York police said the white powder in the envelope was deemed to be nonhazardous and was taken to a lab for more testing. It appeared to contain corn starch, senior law enforcement officials said.

The envelope was postmarked from Boston and was sent to Trump's East 54th Street building.

President Trump and his son both called the act "disgusting."

The Secret Service said Monday it was investigating and would have no further comment.