ISIS has claimed responsibility for the foiled attack, with a bellicose statement, as reported in many media outlets, here.
Again, the combative ideology of radical Islam, not tolerating certain speech or images in public against it, comports more with a group mindset common in the past, where tribes fought each other based on religious affiliation, and took group revenge, rather than directly an objection to western individualism (was we used to see with violent groups on either the radical left or radical right). This really is war over religion, which sounds absurd to many western people, even those who have no trouble reconciling faith with modern physics.
Vox Media has an article by Max Fisher explaining that the gathering was a "hate" event, here.
There is more from CNN on the level of participation from anyone overseas from ISIS here. There are also accounts that the public is lucky that the policeman had not left yet and was able to hit both suspects with his handgun despite their body armor.