JACKSONVILLE, Fla. -

A 29-year-old man was arrested on charges of aggravated child abuse after a 1-year-old Mayport boy was hospitalized in critical condition on Wednesday, according to the Jacksonville Sheriff's Office.

The child was found unresponsive and not breathing Wednesday at Ribault Bay Village Navy Family Housing in the 1000 block of Bunker Hill Court in Mayport.

Police said they questioned the boy's babysitter, Rickey Valentine, and later arrested him. They said Valentine was the only adult home at the time. He has no relation to the child, but was his regular sitter.

Police said the boy had bruising and swelling on the back of his head from ear to ear. He was taken to Shands Jacksonville Medical Center with life-threatening injuries. Police later said the boy was recovering and is expected to survive. His condition was upgraded to serious.

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Medical personnel at the hospital told police the injuries were due to "very severe, non-accidental trauma to the back of head," police said. The baby had "two occipital fractures and a right parietal subdural hematoma caused by violent shaking," police said.

Police said Valentine was babysitting the boy and three other children no older than 3 when he called 911 about 11:30 a.m. because the boy was unresponsive. The other children were unharmed.

Valentine was being held in the Duval County jail with bail set at $75,000.

Police said the baby's mother is in the Navy and was at work at the time her son was hurt. She later went to the hospital to see her child, police said.

The Department of Children and Families is also investigating.

"In situations like these, depending on what the other children may have seen, it's possible that they might have been shocked or disturbed by all the commotion that took place, especially when that child had to be taken to the hospital," DCF spokesman John Harrell said. "So we're going to see whatever services that we can provide."

Channel 4 crime analyst Ken Jefferson said this is a difficult case because Valentine was the only adult at the home, and the other children are likely too young to tell what really happened. But police are confident this was not an accident.

"At this point, police do not think that another child inflicted these injuries on this baby," Jefferson said. "It had to be at the hands of another adult. Otherwise, it would still have to be an ongoing investigation before they could make that determination."