A man who was arrested Monday for allegedly beating a 1-year-old boy was charged Thursday with first-degree assault in Everett District Court.

Matthew P. Christiansen, 31, was being held on $100,000 bail.

Christiansen had been baby-sitting the boy when the child suffered a broken jaw, cracked ribs and a fractured arm. He also had bleeding inside his skull and multiple bruises.

A doctor at Harborview Medical Center in Seattle told investigators the child’s injuries didn’t appear to be accidental. The broken jaw likely resulted from someone punching the child in the face, court records said.

Injuries occurred over time and were not consistent with a fall.

Christiansen told detectives he had been carrying the boy when he tripped and fell, causing the boy to tumble out of his arms and down concrete stairs.

Prosecutors have until Oct. 12 to bring the case to Snohomish County Superior Court, where felony charges are handled.