Losing fish!! Please help me find the Koi Killer!!

I have lost at least five Koi in two weeks!!! and they are gone
without a trace!!! no blood, no parts of body!!
and I am sure they are lost because I could see the bottom of the
pond, and every time I feed fishes, all of them would show up

My Pond is 5' deep (15'x22') & the fishes lost are about 6'' to 8 ''
long,

One strange thing is I have 4 bigger Koi, about 1' - 1 1/2 ' long,
they are OK
I do think it is bird or it would took the bigger fish, maybe the
bigger Koi is smarter?
The neighbor's cat is a constant visitor of the pond, I saw Racoon
twice
I just hat to net the pond

I suspect that because I have rock on the pond edge, the predictor may
caught the Koi when it was feeding the algae close to the rock.
I am thinking about net the edge of the pond (1' from the edge to the
water (4 sides), and leave the center open)
Please advice
Thanks in advance
John