User blog comment:PinkuNeko/Human transmutation./@comment-88.112.224.119-20130530062332/@comment-1618178-20130603143745

That's the thing: they didn't transmute their mother's body... they transmuted A body, in the hope that their mother's soul could be attached to it.

But the thing is: since their mother is dead, where's her soul at? Not inside the Gate! And what is the mechanism that they could have that would access the soul of Trisha and bring it back to the living?... None.

The whole question about Human Transmutation is that Father has always fooled all Amestrian Alchemists into thinking that Human Transmutation is a sort of transmutation used to ressucitate people when, in truth, its something else entirely - namely, means to access the Gate of Truth and adquire Wisdom and Power. - All the while, both Father and Hohenheim were aware of the truth: that you cannot bring a person back to life because the link between a soul and a body is unique and cannot be redone once it's been broken (by death) and, therefore, humans have no access to the "place where souls rest" to be able to reconnect a soul to another body. - that is why Hohenheim 'implied' to Ed that it was impossible: to show his son that Human Transmutation is some different than Ed thought it was.

That, in fact, could be one of the interpretations of the "The Truth behind the Truth".

So what happened in that transmutation in particular is that the body requires a soul and, since it cannot connect itself to a soul from a dead person, it'll connect to the one that is "available" nearest, attracting that soul into itself. That's why Al's soul was temporarily inside the "mom's body".