BIRCHWOOD, Wis. – As several deer hunters made their way through the woods of northern Wisconsin, they were startled to come upon a stranger in their tree stand. But what happened next was even more astonishing.

Asked to leave, the trespasser, wearing blaze-orange and carrying a semiautomatic assault rifle, opened fire on the hunters and didn’t stop until his 20-round clip was empty, leaving five people dead and three wounded, authorities said. The shooter was eventually captured.

The killings baffled authorities and stunned residents in a state where deer hunting is a rite of autumn practiced by thousands of Wisconsinites.

“This is an incredible tragedy, one in which a great family tradition like a deer hunt has turned into such a great loss,” Governor Doyle said yesterday.

Police identified the shooter as Chai Vang, 36, a hunter from St. Paul, Minn., who is a member of the Twin Cities’ Hmong community.

While authorities do not know why he allegedly opened fire, there have been previous clashes between Southeast Asian and white hunters in the region.

Locals have complained that the Hmong, refugees from Laos, do not understand the concept of private property and hunt wherever they see fit.

The five killed and three wounded were part of a group of 14 or 15 who made their opening-weekend trip an annual tradition.

The gunman was “chasing after them and killing them,” Deputy Tim Zeigle said. “He hunted them down.”

It is unclear whether anyone returned fire. The members of the hunting party had only one gun among them. Killed were Robert Crotteau, 42; his son Joseph, 20; Al Laski, 43; Mark Roidt, 28; and Jessica Willers, 27.