The charges were filed against Robert Okerstrom, 26, who was arrested in July shortly after the child received medical treatment at a Seattle hospital.
A doctor concluded that the broken arm was "highly suspicious," court records said.
Okerstrom initially speculated to police that the broken arm could have occurred accidentally when he was trying to remove the child from his car seat, court papers said.
Later, he allegedly admitted bending the baby's arm behind his back while in a "fit of rage," prosecutors wrote. He reportedly told a detective that he thought he might have hurt the child but, upon checking the baby's arm, "didn't think anything was wrong or worthy of medical attention."
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