Help required

Dear all,

I live in the south west of the UK. My neighbour has had her outdoor
pond invaded by some type of creature that has killed nearly all her
Koi.
She heard a lot of splashing in the pond at 3.00am but was affraid to
go out. This morning she has found all the Koi scattered around the
garden with all the heads missing!!!!!
What on earth would do this?. We have had problems with herons in the
past but not where they left the fish with heads missing.
My neighbour has now put a net to protect the remaining fish left but
we need to know what creature would do this??
Thanks in advance,
Gary.