Donald Trump Jr.'s wife has been taken to hospital as a precaution after opening an envelope that contained white powder and feeling ill, the NYPD said on Monday.

A police spokesperson said a preliminary test of the powder indicated it wasn't dangerous.

The New York Police Department said U.S. President Donald Trump's daughter-in-law, Vanessa Trump, called 911 Monday morning after opening a letter addressed to her husband at their midtown Manhattan apartment. Police said she reported coughing and felt nauseous.

The Trump Organization didn't immediately respond to a request for comment, and it is still unclear what the powder was.

Detectives from the NYPD's intelligence division and Secret Service agents are investigating. A spokesperson for the Secret Service hasn't responded to a request for comment.

Police said a hazardous materials unit is also at the scene.