Suspect in deadly Toronto van attack limited presence on social media, but once cited 2014 Isla Vista mass shooting—in which Elliot Rodger killed six people near UC Santa Barbara—on Facebook. https://abcn.ws/2Fe0A5P pic.twitter.com/imlPlgjGz5
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That seems to be the current theory, yes. He's an "Incel Rebel" Incel being short for "involuntary celibate"