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.

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.”

White House press secretary Sarah Sanders said the President had spoken with his daughter-in-law on Monday afternoon, and Trump Jr.’s sister and White House adviser Ivanka Trump addressed the incident on Twitter.

“Thinking of @MrsVanessaTrump & wishing I was by her side today. No one deserves to be frightened this way. There is no excuse,” Ivanka Trump tweeted.

The Fire Department of New York said three people were transported to a local hospital with minor injuries, but did not provide additional information on the nature of the injuries or the victims.

The Secret Service acknowledged that it was participating in the investigation but declined further comment.

CNN’s Jim Acosta contributed to this report.