Police say Officer Michael shot and injured McCarthy during their exchange of gunfire that ultimately killed Michael.
Missouri State Highway Patrol Sergeant Bill Lowe says Officer Michael was able to return fire hitting McCarthy.
Lowe says he couldn't immediately specify the style of the rifle.
He is scheduled to be arraigned Friday on charges of first degree murder and armed criminal action.
William Noble, 35, has been charged with tampering with evidence in felony prosecution.
Missouri Highway Patrol officers searching for suspect Ian McCarthy prepare to drive into a field parked with old mobile homes on the outskirts of Clinton, Mo., earlier this week.
Police say he then changed his story and admitted he bought the gun for McCarthy who had asked him to do so. They say there were three weapons upstairs and a gun safe downstairs.
When Noble asked what kind of weapon was used to kill the officer, an investigator with the highway patrol told him, "He knew the answer to this question", according to the affidavit. He took the two officers to the creek Thursday, where they recovered a rifle that "meets the description and appears to me to be the weapon used in the murder", according to the probable cause statement.
He then told them that he had been out Sunday night, but when he came home about 2 a.m. Monday he found the rifle inside the doorway. Noble said he panicked, took the gun and drove north out of town.
The court documents indicate Noble gave police a written statement about buying, then later selling the gun to McCarthy.
Michael and McCarthy had exchanged gunfire before the officer died and the driver fled. Investigators on Wednesday still were trying to retrace McCarthy's whereabouts during his two days on the run and determine if he had any help eluding authorities, Lowe said.
Investigators have said it's possible that other people helped McCarthy by giving him a ride or clothes.