Robin Westman — who killed 2 kids and injured 17 people in a targeted shooting at a Catholic church in Minneapolis Wednesday — reportedly wrote a long-winded manifesto and uploaded a number of strange videos prior to the attack.

According to CNN … investigators are currently examining — and trying to authenticate — YouTube videos allegedly posted by the shooter in order to gain more insight into a possible motive for the violence.

The videos in question — and the account itself — have since been taken down, but Minneapolis Police Chief Brian O’Hara told reporters Westman had a “manifesto” scheduled to be published on YouTube Wednesday … when authorities say Westman shot into morning Mass at Annunciation Catholic Church with a rifle, a shotgun and a pistol.

Westman also reportedly expressed a “deep fascination” with infamous mass shooters — including Sandy Hook shooter Adam Lanza.

Authorities hope to uncover a clear motive from this troubling material. The notebook apparently features passages indicating state of mind, including … “I don’t want to do it to spread a message. I do it to please myself. I do it because I am sick.”