Third Man in the Graveyard?

[pippin_999]

No: HPFGUIDX 85779

The text of OOP doesn't really tell us who killed Sirius.  It occurs 
to me  that we don't know who killed Cedric either. 

Harry is on the ground with his hands over his eyes when Cedric 
dies in GoF. It was Voldemort's wand, certainly, but all we know 
about the speaker of the curse is,

***
A  swishing noise and a second voice, which screeched the 
words to the night: 'Avada Kedavra!'
***

When Harry opens his eyes again, Wormtail has put down his 
bundle and drawn out a wand. This wand, which Peter uses to 
bind Harry and light the fire at the base of the cauldron, is 
apparently not the wand which killed Cedric, since there is no 
echo of these spells during the priori incantatem scene. 

So maybe, just maybe, there is another Death Eater present, out 
of Harry's field of view, which JKR tells us several times is very 
limited.

Pippin
who has no evidence to prove that ESE!Lupin was the twentieth 
hijacker or the second gunman. <g>