"Mr. Morgan was the initial aggressor in the encounter, Mr. Morgan threatened the use of deadly force against the officer, and Officer Cook acted in the reasonable belief that deadly force was necessary to defend himself from Mr. Morgan's actions," Mittelhauser wrote in a statement.
Mittelhauser said the conclusion was easy to reach and was "not a close call."
Police Chief John DeGonia said Cook was reinstated to full duty. He had been on paid administrative leave during the Highway Patrol's investigation. Cook will return to patrol during his regularly scheduled shift on Saturday.
DeGonia said the department has an "outstanding training program" that is recognized by the police community, and Cook acted well within the police policies and training.
The Highway Patrol took written witness statements during its investigation, and compiled a 3-inch thick report.
"The only thing we investigate on is the shooting itself," said Sgt. Scott Meyer, a spokesman for the Highway Patrol. "We present the facts of the case to the prosecuting attorney."
Meyer said it is up to the prosecutor to file charges, call for a grand jury or coroner's inquest, or find no wrongdoing. "We don't make any conclusions," he said. "We investigate it, and report it; nothing but the facts."
Sgt. H.A. Fowler, who investigated the incident, classified it as an attempted assault on a police officer in his report. In his summary, David Morgan is listed as the suspect and Cook the victim. Fowler was unavailable for comment Thursday.
According to the Highway Patrol report, police were called by a relative of Morgan's girlfriend, Angeline Jackson. Jackson and Morgan had argued, and she told him to leave and threatened to call police. Morgan went outside and cut the telephone lines. Several witnesses said Morgan was drunk.
Jackson told Fowler that she locked the door when Morgan went outside. She went onto the porch and stood by the doorway when Cook arrived, the patrol report stated.
Morgan was sitting in a chair on the porch. Jackson warned Cook that Morgan had a knife. With his gun drawn, Cook ordered Morgan several times to put down the knife.
The patrol report stated that Morgan cursed at Cook and said, " 'What are you going to do if I don't drop the knife?' Officer Cook said he told him, 'I'm going to shoot you!' Officer Cook said Morgan replied, 'F--- you, shoot me, I'm going to die anyways.' "
Cook said in his statement to Fowler that he thought he might be able to subdue Morgan and tried to get his pepper spray from his belt. Morgan stood up from the chair and "took an aggressive stance on the edge of the porch," according to the report.
"Officer Cook said he believed an attack was imminent, so he abandoned attempts to pull the (pepper spray) from its holder," Fowler wrote.
The officer told Morgan several times to drop the knife. Morgan yelled "F--- you," then lunged at Cook, according to the report. The officer fired a single shot to Morgan's lower chest, then holstered his gun and began cardiopulmonary resuscitation on Morgan.
Statements from Jackson, and her son, Andrew Jackson, 23, supported what Cook told authorities.
"He should have done what the officer said and dropped the knife," Angeline told investigators.
DeGonia said Cook saw an Kansas City psychologist specializing in police shootings, who cleared the officer to return to work.
The Pettis County chapter of the National Association for the Advancement of Colored People declined to comment on the investigation until it can review a complete report, said President Rhonda Chalfant. She said members were getting a report.
