why are they dying?

So...it's not really an emergency at this point..but I suppose it could become one...

The 411 is this:

shipment of 50 chicks
feeding medicated start and grow by purina
also feeding that to 20 other SEPARATELY KEPT chicks - healthy I may add
1 arrived dead (came on Wednesday)
7 more dropped dead the next night sometime
doing everything for the new chicks as sanitary as possible (i.e. not contact with other prior to feeding them, etc)
1 more yesterday found just laying there, seemed to be struggling to breath, but no discharge from nose, died in the night (had moved it
to it's own warm spot away from EVERYONE)
1 more today not looking so hot - just lays there, legs in a normal position, except when you pick it up, it doesn't move them at all - even the wings are just kinda kicked out to the sides immobile - this one however is still peeping, can't decide whether to cull or leave it be


What is causing this? Why are they just going limp and paralyzed? I tried searching and kept coming up with spraddle leg - but that doesn't really fit the symptoms...any ideas? DH is heart broken, and I have no answer for him...

 

all the others seem to be doing fine - moving around, eating, drinking, etc...it's hovering between 95 under the light and 80 on the far side of the brooder - idk....just hoping it's not some disease I haven't heard of yet. Only had one pasty butt in the whole crowd and that ones doing great now...just at a loss as to why so many losses in this one group