An envelope mailed to Donald Trump's campaign office inside Trump Tower contained a suspicious powder, prompting an emergency response as authorities worked to determine whether it was hazardous, police say.

Police said a worker opened an envelope mailed to the Republican presidential candidate's campaign office on the fifth floor of Trump Tower at 725 Fifth Ave. in midtown Thursday evening and found a white powdery substance inside.

Six people were in the room when the envelope was opened, according to police, but only three men were in direct exposure of the powder. Those men were evaluated inside the building.

FDNY and NYPD responded to the scene, along with Homeland Security.

Authorities tested the powder and determined a few hours later it was non-hazardous. Additional testing was needed before it could be officially identified.

Trump was campaigning in California Thursday evening. A message left with his campaign team was not immediately returned.

In March, Trump's 32-year-old son Eric Trump was sent an envelope containing a suspicious powder and threatening note. The powder was deemed non-toxic but the note threatened the younger Trump, saying "If your father does not drop out of the race, the next envelope won't be a fake."

Two days later, a threatening letter was sent to Trump's sister Maryanne Trump Barry, a judge who sits on the U.S. Circuit Court of Appeals for the 3rd Circuit in Pennsylvania. The FBI said at the time it was working alongside the Secret Service and the Marshals Service to investigate.

Powders sent through the mail have been cause for concern since at least 2001, when anthrax-tainted letters were sent to media outlets and offices, killing five people.

Trump, the front-runner for the Republican presidential nomination, has been criticized by some political rivals and voters for his comments on topics including women, refugees and immigrants, such as when he said some Mexican immigrants in the U.S. illegally are "rapists."