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By Tom Winter and Jonathan Dienst

The suspicious letter that sent President Donald Trump's daughter-in-law to a Manhattan hospital was postmarked from Boston and appeared to contain corn starch, multiple senior law enforcement officials told NBC News.

The letter was addressed to Vanessa Trump's husband, Donald Trump Jr., the president's eldest son, officials said.

The Trump family expressed disgust over the Monday incident.

"No one deserves to be frightened this way," Donald Jr.'s sister Ivanka tweeted.

Donald Trump Jr. called it "disgusting" in a tweet but said everyone was safe.

Vanessa Trump and two other people were taken to New York Presbyterian-Weill Cornell Medical Center strictly as a precaution.

The NYPD said the white powder in the envelope was deemed to be non-hazardous and was taken to a lab for further analysis. More testing is being done, but officials said it appeared the substance was harmless corn starch.

"How disturbed must a person be to do what they did to a mother of five children?'' Michael Cohen, President Trump's personal attorney, said in a statement. "This dangerous and reckless act goes beyond political differences."

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

Residents who exited the 54th Street building were shocked by the incident.

David Schnall, a 52-year-old lawyer who lives in the building, called the incident "horrible," and said he's "upset it happened."

Schnall also said, "The media is inflaming a lot of hysteria with regard to Trump and this is just a product of that. There is an inflammation of hatred towards the president and anything to do with him, and hopefully this will be a signal for people to stop."

Lois Winston, a psychologist who works and lives in the building, said she doesn't know any of the affected residents, but she feels "sorry for them" and wishes them well.

"I think it's terrifying," she said. "I think it's terrifying that anybody would send a package like that, regardless of who it was to, it's terrible. But there are so many terrible things right now that it's not surprising."

Yelena Dzhanova contributed.