The letter, which arrived at Eric's Manhattan apartment on Thursday (17Mar16), calls on the business tycoon to quit his run in the Republican Party's nomination race.

An investigation has now been launched after Eric's wife, CBS news reporter Lara Yunaska, 33, opened the suspicious letter, which warned that unless his father withdrew from the contest harm would come to 69-year-old Donald's five children.

According to the New York Daily News, the New York Police Department sent out an emergency response team of more than 24 officers to the scene.

A NYPD spokesperson confirmed the news, detailing what had happened.

"At approximately 7:15 p.m., the NYPD responded to a residential building at 100 Central Park South to investigate a report of a suspicious letter received by a tenant. The letter has been removed and is being examined by law enforcement authorities," the spokesperson said.

Following tests the police authority revealed to ABC News the mystery substance, which was allegedly sent from Massachusetts, was harmless.

The news comes as hacker group Anonymous announced "total war" on the Republican candidate after leaking Donald's phone and social security numbers

Eric, 32, is Donald's second son to socialite and former model Ivana Trump, 67, and has been a regular feature on his father's campaign trail. He was at Donald's side when he won the Florida primary this week (15Mar16).

He is also employed by his father's real estate business as president of development and acquisitions.

Donald , who is known for his outspoken views, is no stranger to controversy and is becoming something of a cult hate figure among A-list celebrities for his hard line views on immigration and alleged sexist remarks, once remarking in a New York Times interview: “I think the only difference between me and the other candidates is that I’m more honest and my women are more beautiful."

Among the stars who are supporting the Stop Hate, Dump Trump campaign are Kerry Washington, Lily Tomlin and George Clooney.

Actor George recently remarked to British newspaper The Guardian, "He's (Trump) just an opportunist. Now he's a fascist; a xenophobic fascist."

He is currently leading the Republican party race.