Police charged an 18-year-old Hilo man with first-degree assault Friday for allegedly causing serious head trauma to his 4-month-old son.

Kainoa C.E. Poepoe-Vigil is being held in the Hilo police cellblock in lieu of $100,000 bail. His initial appearance is scheduled for Monday afternoon at South Hilo District Court.

According to police Capt. Randall Medeiros, fire rescue personnel took the baby, who was unconscious and unresponsive, to the Hilo Medical Center emergency room at about midday Wednesday. Hospital personnel notified police, Medeiros said, and officers arrested Poepoe-Vigil that afternoon.

"He was taken into custody without incident at Hilo Medical Center," Medeiros said.

The boy was later flown to The Queen's Medical Center in Honolulu, where he was listed in critical condition.

Medeiros said subsequent investigation indicated that the father was responsible for the baby's injuries, but he declined to say how the injuries were caused.

Medeiros said the boy's parents are an unmarried couple, but declined to disclose the mother's name or age, or if they have any other children.

The mother has not been arrested or charged in the incident, but she and other family members have been interviewed by detectives, Medeiros said. He declined to say whether charges are being considered against the mother, citing the ongoing investigation. Police are asking for the previously mentioned witnesses and anyone else who may have seen the assault to call the police non-emergency number at 935-3311. Those who wish to remain anonymous can call Crime Stoppers at 961-8300. All Crime Stoppers information is kept confidential.

"No matter how long you've been on this job, there are some things that still affect you," Medeiros said. "This case is particularly hard for all of us. Some of our people have children around the same age as this boy.

"We're hoping and praying for his recovery."