Matiss was at an Independence Day party at a private residence in Novi, Michigan when the deadly accident occurred.
Officials in Michigan say a mortal style firework was inadvertently fired in the direction of a group of people in a hot tub ... striking Kivlenieks.
Rescue workers rushed to the scene around 10 PM and transported Matiss to the hospital ... but doctors were unable to save the goalies life.
Law enforcement is still investigating ... but don't believe there was any foul play involved.
“At the moment, we’re pretty certain this was a tragic accident,” a police spokesperson said.