Heart attack killed man in NT lane

A man found dead in a Darwin lane after a fight with a relative suffered a heart attack, an autopsy has found.

The 48-year-old man was found dead in a laneway near his home in Karama two hours after he left the house after an argument with a 22-year-old male relative late on Tuesday night.

The 22-year-old had been in police custody since Wednesday assisting with their investigation but has since been released, AAP understands.

Police don't suspect the men were involved in any criminal activity.

"This is an isolated incident, it's a tragic outcome," Detective Senior Sergeant Lee Morgan said.