By Brynn Gingras and Sarah Jorgensen, CNN

(CNN) -- President Donald Trump's daughter-in-law opened a letter containing an unknown substance that was addressed to the President's son, the New York Police Department said.

The letter was sent to the couple's Manhattan apartment and opened around 10 a.m. ET Monday, authorities said. Vanessa Trump did not have an immediate reaction to the substance, which is being tested, the NYPD said.

The Fire Department of New York said three people were transported to a local hospital with minor injuries, but did not provide additional information on the nature of the injuries or the victims.

A message left with the Secret Service was not immediately returned.

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