(AP) -- An envelope containing a suspicious white powdery substance caused a scare when it was opened at a campaign office of Republican presidential candidate Donald Trump, but it later was deemed to be harmless.

The envelope was mailed to Manhattan's Trump Tower, near Central Park, police said. A campaign staffer opened the envelope Thursday night and immediately called police.

Five Trump staff members working in the office and a police officer who responded were temporarily isolated and evaluated. The substance was tested, and a few hours later authorities said it wasn't hazardous but it would need to be tested some more for them to determine what it is.

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."