my koi ate my goldfish!!!

i recently brought in 5 small koi about 4 inches in length also in the
tank were 5 small goldfish that survived from my hatch this year, (my
first ever). the gold fish were about an inch long. well after two days
in the tank i realise their is a goldfish floating on the surface bitten
in half and still alive! the others had all gone, i was not happy due it
being my first baby goldfish and the fact i had put alot of effort into
rearing them.
why did they do this, has anyone had similar experiances????

matt


--
matt