NEW YORK -- President Trump's daughter-in-law was taken to the hospital Monday after opening an envelope containing a "suspicious" substance addressed to her husband, Donald Trump Jr. The New York City Police Department (NYPD) said initial tests did not indicate the powder was harmful but they are continuing to test the substance. Law enforcement sources later told CBS News the substance turned out to be corn starch.

Vanessa Trump and two others, including her mother, were taken to a nearby hospital as a precaution. The NYPD said officers and Secret Service agents responded to the couple's apartment in midtown Manhattan on Monday morning around 10 a.m.

CBS News senior investigative producer Pat Milton reports that Vanessa Trump, 40, wasn't hospitalized but was tested at the hospital out of an abundance of caution.

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Donald Trump Jr. later tweeted he was thankful his wife and children were safe.

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

Vanessa Trump's mother, Bonnie Haydon, handed her the envelope, which Vanessa then opened, CBS New York reports. The identity of the third individual taken to the hospital was not immediately clear.

The Secret Service said it is "investigating a suspicious package addressed to one of our protectees received today."

Ivanka Trump addressed the incident on Twitter saying she wished she was by her sister-in-law's side.

Thinking of @MrsVanessaTrump & wishing I was by her side today. No one deserves to be frightened this way. There is no excuse. — Ivanka Trump (@IvankaTrump) February 12, 2018

In March 2016, law enforcement officials investigated a threatening letter addressed to Donald Trump Jr.'s brother, Eric. The letter also contained a suspicious white powder that turned out to be harmless.