OKLAHOMA CITY - The FBI is calling a serious of suspicious letters sent to numerous Oklahoma government buildings this week a "hazardous material hoax."
Thursday, news broke of an FBI, Secret Service and Office of Hazardous Materials Safety investigation at three federal buildings in downtown OKC--the U.S. Attorney's Office, the federal courthouse and FBI headquarters.
READ: 'Investigators looking into suspicious packages' (http://bit.ly/10SbswG)
The FBI confirmed Friday the letters were sent from an Oklahoma prison inmate who falsely indicated the white powdery substance in the letters was anthrax. The bureau's preliminary lab tests have ruled out any harmful substance on the letters.
The FBI also said the inmate had mailed a letter to the U.S. Attorney's Office for the Eastern District of Oklahoma in Muskogee, which they received Wednesday, and the U.S. Court of Appeals in Washington D.C. That letter was intercepted by the U.S. Postal Inspector's Office Friday, according to the bureau.
The FBI has not released the name of the inmate or the contents of the letters.