Former Arbroath SC and Tayport FC defender Peter McAvoy, from Dundee, collapsed and died at his apartment in Herkimer, New York State last month at the age of just 22.
His father, also Peter, was initially told his son had probably died from cardiomyopathy, a disease of the heart that can be genetic.
But Mr McAvoy, who still lives in Dundee, has now been told that his son - who was the captain of his college team Herkimer Generals - died from Acute Virai Myocarditis, an inflammation of the heart muscle, and experts have no idea what caused it.
He said: “It’s just one thing after another.
“The whole process of going to America, bringing his body home and then finding out he died of something for a completely unknown reason is just horrible.
“The sad thing is that this virus doesn’t necessarily kill everyone, it just happened to attack Pete’s heart and he had no chance.
He added: “I will keep trying to find out what happened. I want to find out why my son died.
“The only positive I can take right now is that some of his pals are now raising money for the British Heart Foundation, which is a great gesture.”