The US president’s daughter-in-law, Vanessa Trump, has been taken to hospital after opening an envelope containing white powder.
The letter was addressed to Donald Trump Jr, Mr Trump’s eldest son, at his apartment in Manhattan.
She and two other people at the scene were decontaminated by firefighters and taken to hospital for evaluation.
The Secret Service is investigating, and officials told the BBC that Mrs Trump appears to be in good health.
The Fire Department of New York (FDNY) referred calls from the media to police, but confirmed that three people had been transported to the Weill Cornell Medical Center.
In September, Mr Trump Jr chose to forgo Secret Service protection for himself, his wife and his five children, but they were reactivated one week later.