Supreme Court spokesman Bill Gang confirmed to the Associated Press that the letter was opened on Thursday, and tracked to an inmate held in either the High Desert State Prison or Southern Nevada Correctional Center. Both facilities are located in Indian Springs.
Because the incident involves the U.S. Postal Service, the FBI will handle the investigation. The FBI and the U.S. attorney's office in Las Vegas declined to comment.
Gang said the inmate who sent the powder does not have dealings with the court. The inmate's name has not been released.
"He has no case pending before the Supreme Court and has not had a case before the Supreme Court," Gang said, according to the Associated Press.
Four court employees were near the envelope when it was opened on Thursday, and they were isolated as a precaution. The letter claimed the enclosed substance was anthrax, but a hazmat team was able to determine the powder was not hazardous, Associated Press reports.
Outgoing mail from a corrections facility is not opened or inspected except in the event of suspicious or prohibited activity.