Well, it can be translated as 'of' if you want to do it the "English" way.

A criminal case of gas exposion has been started.

Anyway, по in Russian is more about to a movement to/on the surface of something:

Like

ударить по плечу
идти по дороге

In other cases it can be translated as "on/about"

лекция по философии - a lecture on philososphy