Did Peter really kill those Muggles?

[pippin_999 <foxmoth@qnet.com>]

No: HPFGUIDX 49360

In PoA 19 Sirius says, "Then before I could curse him, [Peter] 
blew apart the street with the wand behind his back, killed 
everyone within twenty feet of himself -- and sped down into the 
sewer with the other rats."

Unless Sirius has X-ray vision, he couldn't have *seen* the wand 
behind Peter's back, so this must be conjecture. Given that Peter 
is supposedly such a weak wizard, could he have had a 
co-conspirator who blew the street apart instead? 

Pippin