Authorities say the Muscogee (Creek) Nation executive building in downtown Okmulgee was evacuated after a suspicious piece of mail was delivered late Wednesday morning.
Officials said a substance found in the letter was determined to not be hazardous and emergency crews cleared the scene at 1:15 p.m.
Tribal officials closed the building and evacuated the offices of Principal Chief George Tiger, Second Chief Roger Barnett and all other personnel in the building.
According to the Okmulgee Daily Times, Creek Nation Lighthorsemen were dispatched along with the Okmulgee Fire Department, Okmulgee County EMS and Creek Nation Emergency Management.
Tribal officials said the FBI is investigating the letter and who may have sent it.