How does Mathematica obtain this result?

FullSimplify[  Sqrt[2 \[Pi]] InverseFourierTransform[1/(x^2 - a^2), x, p],   Element[a, Reals]] 

Gives the output

-((\[Pi] Sign[p] Sin[a p])/a) 


$ \int_{-\infty }^{\infty } \frac{e^{-i k x}}{x^2-a^2} \, dx$ is not a defined integration. Mathematica also returns undefined as answer if you compute it.

So, I am trying to understand how does Mathematica calculates that Fourier transform of the non-integrable function.