Cook then allegedly fired the gun multiple times at 26-year-old Lisa Vilate Williams while her daughters were watching.
The victim was shot twice in the torso and fell back onto the couch.
Her ex-husband told police that Cook sat in a recliner chair near their daughters and watched as he tried to help his injured girlfriend.
When she got up and tried to get her coat, her ex-husband pinned her to a wall until police arrived.
Williams was pronounced dead after she was rushed to hospital.
Cook, who was arrested on suspicion of aggravated murder, taught health class at Skyridge High School.
She is also currently facing separate domestic violence charges related to an incident in October to which she has pleaded not guilty.
The charging documents in that case are sealed but it is related to abuse in front of children.
No attorney was immediately listed for her in court documents.
