New York (CNN) President Donald Trump's daughter-in-law opened a letter containing an unknown substance that was addressed to the President's son Donald Trump Jr., the New York Police Department said.

The letter was sent to the couple's Manhattan apartment and opened around 10 a.m. ET Monday, authorities said. Vanessa Trump did not have an immediate reaction to the substance, which the NYPD said was deemed nonhazardous.

Trump Jr. said on Twitter that his wife and children were unharmed.

"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," Trump Jr. tweeted.

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 sent a tweet Monday evening thanking organizations for responding to the incident, writing: "Thank you so much for all the help today in NYC! I appreciate all the quick response to make sure that I was safe ! Thank you @FDNY @SecretService @NYPDnews @NYPDCT @NewYorkFBI."

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