NEW YORK (CBSNewYork) — Police and the FBI are investigating a suspicious piece of mail that was sent to Donald Trump’s son, Eric.

Police were seen outside Eric Trump’s residence at 100 Central Park South early Friday morning.

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Authorities swarmed the building, known as Trump Parc East, on Thursday after sources told CBS News that a threatening letter was sent to the Republican presidential candidate’s middle son.

Investigators said the letter, postmarked from Massachusetts, was opened by Eric Trump’s wife Lara Thursday night. Sources told CBS News that a white powder was found inside of the envelope.

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Lara Trump called police just after 7 p.m., triggering the response by the FBI, the NYPD and the FDNY.

Sources said the enclosed letter warned that if Donald Trump does not drop out of the presidential race, his children could be harmed, CBS2’s Janelle Burrell reported.

But a source close to the investigation said preliminary tests show that the white substance does not appear to be hazardous. Further tests are being conducted at a crime lab, WCBS 880’s Sean Adams reported.

Police were also seen Friday outside Trump Tower on Fifth Avenue, 1010 WINS’ Al Jones reported. CBS2 reached out to Donald Trump’s campaign for a comment, but has not heard back.

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The investigation is ongoing.