Phoenix, Arizona - A staff member at the headquarters of Republican gubernatorial candidate Kari Lake opened a letter containing a white substance over the weekend, prompting an investigation by the Federal Bureau of Investigations (FBI).

With midterms quickly approaching on Tuesday, the politician's office has been shut down since Saturday as law enforcement investigates the mysterious incident.

"A member of the Kari Lake staff opened an envelope delivered to our campaign office that contained suspicious white powder," Colton Duncan, a spokesperson for the campaign, told ABC News.

"It was one of two envelopes that were confiscated by law enforcement and sent to professionals at Quantico for examination, and we are awaiting details."

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The staffer did not open the second letter, which was identical to the first.

According to Phoenix police sergeant Phil Krynsky, officers were joined by a hazmat team, a bomb squad, and the FBI to respond to the "found property call."

"There have been no reports of injury and the investigation remains active," he added.

The campaign described the incident to the Daily Mail as a "terroristic attack, trying to intimidate Kari and the campaign staff" and said a letter was included with the substance that had "a bunch of vulgarities and ranting and raving about Kari."

Lake is in a tight race against her Democratic opponent Katie Hobbs. Arizona will decide who will be governor this Tuesday.