NEW YORK -- The Secret Service, FBI and New York Police Department are investigating what sources tell CBS News is a threatening piece of mail sent to Donald Trump's son, Eric Trump.

The mail, sent to Eric Trump at his posh Manhattan residence at 100 Central Park South, also known as Trump Parc East, was opened by his wife Thursday.

A source close to the investigation said the letter had a Massachusetts postmark and warned that if Donald Trump doesn't withdraw from the race for the Republican presidential nomination -- paraphrasing -- harm will come to the kids.

Investigators in crowded lobby of 100 Central Park South in Manhattan late on March 17, 2016 after what sources say was threatening letter was received by Donald Trump's son, Eric Trump CBS News

A source familiar with the matter said the envelope contained suspicious white powder that's been sent to a lab for testing. The source said a preliminary field test indicated the substance didn't appear to be hazardous.

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No injuries were reported.