It happened in the town of Farmington, west of Fayetteville, just after one o'clock this afternoon.
The man's name has not yet been released.
Washington County Sheriff Tim Helder says detectives from his office were following up on a report of the sexual assault of a minor when they went to the man's home at 12571 Leam Blvd.
Sheriff Helder says detectives were in the process of having him step outside when the man said he needed to go back into the house for something.
While Detective L. Scott McAfee told the man to wait, he stepped inside the house anyway, slamming the door behind him. When Detective McAfee knocked on the door again, the man allegedly swung the door open and was armed with a crowbar he then swung at the detective.
That's when the detective shot the suspect once. He was pronounced dead at the scene.
The Arkansas State Police has been called to investigate the incident. The Sheriff's Office will also conduct its own internal investigation. As is standard procedure, Detective McAfee is on administrative leave pending the outcome of the investigation.