A 17-year-old Coupeville boy accused of assaulting and seriously injuring a man with a baseball bat was arraigned Tuesday as an adult in Island County Superior Court.

Prosecutor Greg Banks said he charged the teenager, Ryan Marti, with first-degree assault because of the severity of the victim’s injuries. The 22-year-old man suffered a skull fracture and hearing loss in one ear.

Banks explained that the charge requires mandatory jurisdiction in adult court — where the sentences are more serious — but the case would be remanded back to juvenile court if both the prosecution and defense agree. The prosecutor said it’s too early for him to make the decision since the investigation is ongoing and he’s awaiting medical reports.

Coupeville Marshal Dave Penrod said the alleged assault occurred in the aftermath of a party on NE Kinney Street on Saturday, April 10. A couple of brothers who had recently moved from the Tacoma area to the Coupeville area went to the gathering and annoyed other people by asserting that parties on Whidbey Island were less than ideal.

The brothers, a 22-year-old and a 16-year-old, left the party and later returned. They started arguing with a couple of other partygoers outside the house. One of the young men at the party pushed the younger brother to the ground and started hitting him, court documents state.

The older brother, Stephen Fanelli, tried to help his brother. Penrod said Marti came up behind Fanelli and struck him over the head with a baseball bat.

“Witnesses said he just came out of the blue with a baseball bat,” Penrod said.

The police report had no description of the type of bat used.

Fanelli was transported to Whidbey General Hospital, but then airlifted to Harborview Medical Center in Seattle. He suffered a skull fracture and possible permanent hearing loss on one ear due a “cochlear ear bone fracture,” the police report states.

If Marti is tried and convicted of the charge as an adult, he could face up to 12 years and three months in prison under the standard sentencing range.

Marti is being held in juvenile detention on $50,000 bail.