Officer Chansey McMillin gave chase, his body-cam capturing his shadow as he trailed the suspect in the afternoon light of January 17.
Walker, 21, is seen stopping suddenly and bending down to pick up something he dropped on the road. A white car approaches.
The graphic video shows McMillin pulling his pistol, taking aim and unloading five shots. Walker turned around, then started running. He was struck three times, police said. In the video, Walker is seen as he tumbles to the ground, then rolls into a ditch. He died at the scene.
Later, the video shows an emotional McMillin leaning against a police car.
"Why did (Walker) have to do that?" he asked as other officers comforted him.
Since Ferguson, Missouri, teen Michael Brown's death
prompted a national move to put body cameras on all America's police officers, scenes like the one captured in the body-cam video from Muskogee will likely become more common.
President Barack Obama recently announced a $263 million package to help local police departments buy 50,000 cameras. The New York Police Department, the nation's largest, launched a pilot body-camera program in December.
In Muskogee, about 50 of its 88 police officers started wearing body cameras in November, according to department spokesman Sgt. Michael Mahan.
The January 17 incident outside Old Agency Baptist Church is the first officer-involved shooting captured by a body camera.
The video was released Friday after the department showed it to Walker's family and local leaders in an attempt to quell tensions in the community, Mahan said.
"We've seen a lot of departments have a bunker mentality and sort of enclose themselves and not be real open with the public," he said. "We're trying to set a precedent. We think this doesn't allow any speculation to foster within the community and lets all the facts come out."
The encounter began with McMillin approaching Walker outside the church and patting him down. "Relax," the officer is heard saying in the video moments before Walker takes off running. "What are you shaking for?"
Mahan said witnesses told police that on the day of the shooting, Walker had threatened his ex-girlfriend, who was attending a wedding at the church that day. Mahan said Walker allegedly told his ex that he "had a bullet with her name on it."
Moments after the shooting, a man who identified himself as the church's pastor is seen in the video approaching the officer and saying: "Don't shoot no more."
McMillin ordered the pastor to stand back. The officer can be heard on his radio calling for an ambulance. Another officer checked on Walker and picked up something off the ground and tossed it away from the suspect. McMillin is heard saying it's a handgun; another officer said the weapon was loaded. One officer checked Walker's pulse.
The pastor is seen in the video telling other officers that McMillin "did everything right."
The investigation is being handled by the Oklahoma Bureau of Investigation.
McMillin is on paid administrative leave, Mahan said. It was his second officer-involved shooting in six months.
The officer was cleared in the July shooting of a suspect who attacked a man with a knife, according to Mahan. The officer opened fire after the knife-wielding man moved toward him. The suspect survived.
Mahan defended the video's release.
"What we're trying to do is to set a standard for the nation and say that we believe that openness and transparency is the best policy," he said.