Matiss Kivlenieks died on Sunday night during the celebration of the American National Day in Novi, Michigan.
His NHL club, the Columbus Blue Jacket, first wrote on its website that he passed away after injuring his head in a tragic fall accident.
Now the AP writes that the autopsy shows another cause of death.
Hit by fireworks
A forensic doctor in Michigan has now determined that Matiss Kivlenieks died of a fireworks display, which hit him in the chest.
Something went wrong when the fireworks display was lit, which led to it starting to lean and shoot at people in the party.
Kivlenieks was then in a hot tub and tried to escape with several other people.
This reports the AP.
Kivlenieks was hit.
He was taken to hospital where he was pronounced dead later in the evening.