He stabbed her in the neck six times, the fatal wound penetrating her spinal canal, set fire to her house, and left.
The jury was asked to consider whether he was provoked by Ms Dunlea to the point where he lost all self-control.
The jury in the trial of a man who admitted killing his girlfriend is unable to decide whether he is guilty of murder or manslaughter.
After seven hours and 13 minutes of deliberations, the five women and seven men said they could not overcome their disagreement.
MU police questioned Campbell, arrested him and took him to Boone County Jail.