An envelope containing a white powdery substance that was mailed to Donald Trump's campaign office in New York City was opened by an intern from Westchester County, according to broadcast sources.

WCBS is reporting that the intern called police immediately after opening the letter, and emergency crews rushed to Trump Tower in Manhattan around 8 p.m. Thursday.

Authorities said the powder was a "non-hazardous substance." Additional testing was needed before it could be officially identified.

Six people had been isolated and evaluated by medical crews at the scene. They included five Trump staffers and a police officer who responded to the call. The intern's name has not been released.

In March, an envelope containing a non-hazardous white power and threatening letter was sent to the Manhattan apartment of Donald Trump's son, Eric.

Trump campaign spokeswoman Hope Hicks referred questions about the brief scare to the U.S. Secret Service, which didn't immediately respond to a message seeking comment.

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.

In March, an envelope that contained a non-hazardous white powder and a threatening letter was mailed to the apartment of Trump's son Eric Trump, who has campaigned for him. The handwritten note, postmarked from Boston, said: "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.

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."