How did Fortinbras die?

In this question, "Fortinbras" refers not to the character who appears at the end of the play, but rather, that character's uncle, who was also named Fortinbras. This Fortinbras was killed in battle, by Hamlet's father.

HORATIO
Our last king, 
Whose image even but now appear'd to us, 
Was, as you know, by Fortinbras of Norway, 
Thereto prick'd on by a most emulate pride,
Dar'd to the combat; in which our valiant Hamlet 
(For so this side of our known world esteem'd him) 
Did slay this Fortinbras;
(Act I, Scene 1)

Note: Hamlet's father's name is also Hamlet.