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 the NYPD said was deemed nonhazardous.
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.
The Secret Service acknowledged that it was participating in the investigation but declined further comment.