PHELPS COUNTY -- A Rolla baby is in stable condition and his mother's boyfriend facing charges after allegedly shaking the four-month-old.

Sammy Joe Mayberry, 25, faces second-degree assault charges.

Police believe he shook his live-in girlfriend's baby boy.

The woman told police she was on a shopping trip, and when she returned, the baby was lethargic and non-responsive. 

Phelps County authorities say Mayberry admitted to shaking the baby, saying he may not have known his own strength.

The boy was taken to a hospital in St. Louis.