CENTRAL PARK SOUTH, Manhattan (WABC) --Officials are investigating a threatening letter sent to the Manhattan apartment of Donald Trump's son, Eric.
An envelope sent to Eric Trump's apartment in Trump Parc East at 100 Central Park South on Thursday also contained a suspicious white powder. There were no injuries and the official said preliminary tests indicated that the white substance was not hazardous.
The letter, according to a law enforcement official, threatened the family.
"If your father does not drop out of the race, the next envelope won't be a fake," it said. It was signed "X."
The letter was postmarked March 5 from Boston and was opened by Eric Trump's wife, Lara.
"I know Eric, I know his wife, whatever. I live there. It breaks your heart to see something like this happen to such a nice family," said Todd Shapiro, a neighbor.
The FBI is leading the investigation and is working with U.S. Secret Service, Postal Inspector and the New York City Police Department.
Eric Trump is the younger of two adult sons of businessman and Republican presidential front-runner Donald Trump. He has been campaigning for his father.
Currently, the Secret Service protects only Donald Trump. The family was previously briefed on the proper handling of mail.
Also on Thursday, a hacking group called "Anonymous" that posted this video on YouTube, is now targeting Donald Trump and calling him a Nazi.
"Now is the time to unite, to fight fascism, to show that we are very serious about stopping any 4th Reich by the fascist Donald Trump," Anonymous said in the video.
Those hackers claim that they've obtained personal information on Donald Trump and are ready to make his life miserable. Federal officials are now investigating.
"We have attached a gift of sorts, Trump's social security number, cell phone number, and other details that might be able to assist you all in independently investigating this would-be dictator," Anonymous said.
Trump's campaign issued a statement to ABC: "The government and law enforcement authorities are seeking the arrest of the people responsible for attempting to illegally hack Mr. Trump's accounts and telephone information."
A government official confirmed to ABC News that federal authorities are investigating.
Thursday in DC, a group of conservatives met privately in an attempt to stop Trump from getting the nomination.
"I think you'd have riots. I'm representing a tremendous, many, many millions of people," Trump said.
Trump has warned of trouble if he's denied the nomination, his supporters are furious.
"If the Republican party comes in to that convention and jimmies with the rules and takes away the will of the people, the will of the Republicans, Democrats and Independents who have voted for Mr. Trump, I will take off my credentials, I will leave the floor of that convention and I will leave the Republican party forever," said Sam Clovis, Trump National Campaign Co-Chair.
The Associated Press contributed to this report.