A snowboarder -- who was night skiing, at Brighton Ski Resort, Wednesday night -- died after running into a tree, just off a groomed, intermediate run.

But police say he was wearing a helmet...and they want to know if David Sitek, 26, had a medical issue prior to the accident.

An autopsy will be performed on the Sandy man.

Emergency responders say Sitek was alive when they reached him on the mountain, but they were unable to stablize him at the resort's medical clinic.