Investigators are trying to determine if the powder is hazardous but no injuries have been reported so far

The letter, addressed to Donald Trump, was opened by an intern at 8pm

Five Trump staffers and a cop are being evaluated after an envelope of suspicious white powder was mailed to Trump Tower

Hazmat teams and emergency crews surrounded Trump Tower this evening after an envelope of suspicious white powder was mailed to Donald Trump.

Six people, including a police officer, were isolated after an intern at the Tower in New York opened the envelope at just after 8pm on Thursday.

They alerted security after they discovered a mysterious white powder inside. The letter had been addressed to Donald Trump at Trump Tower, officials say.

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Hazmat teams and emergency crews have been called out to Trump Tower after an envelope of suspicious white powder was mailed to Donald Trump

Six people, including a police officer, were isolated after an intern at the Tower in New York opened the envelope at just after 8pm this evening

The inten alerted security, who called the police and fire crews, after they discovered a mysterious white powder inside the envelope

The Republican front runner was not in the building. He was campaigning in California at the time.

Staff resealed the envelope and dialed 911. Firefighters responded toTrump's Fifth Avenue skyscraper, between 56th and 57th Streets within minutes an FDNY spokesman said.

The six people, five staffers and the cop, who were in the room when the envelope was opened are still being evaluated but so far, there are no reports of any injuries.

It is feared the white substance could be dangerous after the infamous anthrax attacks in 2001 that followed September 11.

During that year, powdered anthrax was mailed to several news media offices and members of congress, killing five people and infecting 17 others.

Experts have not yet determined whether the powder found in Trump's letter is hazardous or not but authorities are say they are still investigating the incident.

The letter had been addressed to Donald Trump (pictured giving his victory speech at Trump Tower after sweeping the northwest primaries) at Trump Tower, officials say

Firefighters responded to Trump's Fifth Avenue skyscraper, between 56th and 57th Streets within minutes an FDNY spokesman said

The incident comes barely a month after the billionaire's son Eric was sent a letter with suspicious white powder and a threatening message.

The handwritten note said: 'If your father does not drop out of the race, the next envelope won't be a fake.' It was signed 'X,' reported ABC News.

More than two dozen cops were called out to Trump Parc East, one of Trump's luxury buildings on the southern edge of Central Park on March 17, where Eric Trump and his wife Lara live together.

Lara Trump had called the police after she opened the letter, postmarked from Massachusetts, and the white powder fell out.

The six people, five staffers and the cop, who were in the room when the envelope was opened are still being evaluated but so far, there are no reports of any injuries

Experts have not yet determined whether the powder is hazardous or not but authorities are investigating the incident

Hazmat teams pulled up outside the luxury building in Midtown Manhattan after the call at 8.05pm

After tests, the NYPD reportedly found nothing hazardous.

The letter arrived at Eric Trump's apartment the same day the hacker group Anonymous released his father's phone number and social security number in a targeted cyber attack.

The FBI's Joint Terrorism Task Force in New York have launched a joint investigation with the Secret Service, the Postal Inspector and the NYPD, into the letter.

Eric has been a staunch supporter of his father as he bids to become the Republican nomination for President.