In the climax of Captain America: Civil War (2016), Zemo showed Iron Man how his parents really died, and that the Winter Soldier was responsible.
Afterwards, Iron Man asked Captain America whether he knew about it. Captain America admitted that, while he didn't know that Bucky (the Winter Soldier) was responsible, he did know that Howard and Maria's deaths weren't accidental.
But how? How did Captain America know the truth behind Iron Man's parents' death? How did he know that it was not an accident, but murder? This incident happened on 16 December 1991, and Captain America was buried in ice at that time. Did anyone tell him after he awoke? Am I missing something here?