Grainy surveillance footage from an Oklahoma City fast food restaurant that captured an encounter between two figures in a small corner of the camera frame may not seem like much in the way of evidence.

But it is more than Oklahoma County District Attorney David Prater said he usually has to work with when he is presented with an investigation into an officer-involved shooting.

In a letter dated July 8 to Oklahoma City Police Chief Bill Citty, Prater cleared Master Sgt. Steve Helmer in the May 25 shooting of Derek Prophet, ruling the use of deadly force by the officer was justified. A video released to The Oklahoman Wednesday shows Prophet, a homeless man, lunging at the officer.

"The suspect, Derek Prophet, gave Helmer no viable option to deal with the threat other than a deadly force response," Prater wrote in the letter clearing the officer of any criminal wrongdoing.