Great Falls, MT -

A 21-year-old Rocky Boy man has pleaded guilty to assaulting an 11-month-old boy as part of a plea agreement that would dismiss a second-degree murder charge stemming from the child's Oct. 21 death.

Garrett "Kirby" LaFromboise pleaded guilty to assault resulting in serious bodily injury during a federal court hearing in Great Falls on Tuesday.

Prosecutors said LaFromboise told them he was frustrated because the child would not stop crying so he put his fingers in the boy's mouth to gag him and then punched him in the stomach.

Federal prosecutors identified the victim by the initials K.S.

Cecilia Rose Gardipee pleaded not guilty in December to second-degree murder and assault in the death of K.S. Prosecutors said K.S. also suffered skull fractures, brain injuries, bleeding on the spinal cord and may have been suffocated.