New York City police are investigating a threatening letter sent to Eric Trump, son of Republican presidential front-runner Donald Trump, The New York Post reported.

The letter under question had a Massachusetts postmark and was delivered to Eric Trump, 32, at his home on Manhattan’s Central Park South late Thursday, the Post reports.

White powder fell out of the envelope when Eric Trump’s wife, Lara Yunaska, opened it, the Post reports. CBS News also reported that the letter warned that Donald Trump’s children would be harmed if the billionaire frontrunner did not withdraw from the presidential race.

Police, fire crews and the FBI all responded to the scene.

It was not immediately clear what the substance was.