User blog comment:B214/High School DxD BorN Episode 10 Review/@comment-26268422-20150608231720/@comment-26268422-20150609222656

Just realized another reason it is probably Euclid:  Koneko says Issei's aura and the fakes were identical... so close that if Issei had not been with her she would not have noticed the difference. Enter Boosted Gear... Issei has a dragon's aura because of Draig and Euclid has a replica Boosted Gear that, in the LN:

''' “Unfortunately, we couldn’t copy Ddraig’s soul. However, it became possible to make this replica armour. Except, it became necessary to use several famous Dragons’ souls in order to use this Sacred Gear each time. Due to it being powerful, the cost for it became the worst.”  -  Euclid V16 '''

There is also the resemblance to the explosion of the school building caused by the Stray Magicians prior to Euclids first appearance. But only similar in that they both featured magic explosions at school. Different people and buildings involved here.