The prosecutor job is to look at anything but the most likely cause, in this case arson. And even if the government/justice admit it, then they would try to hid

The prosecutor job is to look at anything but the most likely cause, in this case arson. And even if the government/justice admit it, then they would try to hide the community of the offender(s). Like microcolonel wrote it, there has been quite a few other cases in other churches (including some in Paris) a few months and weeks before. In fact the Vatican counted attacks against churches in 2018 (iirc): one every two days! That fact alone is also not talked about in news outlets (which is very different from the media treatment of other religions building attack). We will probably never learn the truth because its implication is too big for the national security.