Before Alek Minassian killed 10 people, the majority of them women, by driving his van into pedestrians on a Toronto street Tuesday, he posted on Facebook praising mass murderer Elliot Rodger and called for an “Incel Rebellion,” an uprising of men who are angry women won’t have sex with them.

Incel stands for “involuntary celibate,” a term that proliferated on 4chan, an online message board that’s frequently the source of harassment campaigns, according to the Southern Poverty Law Center (SPLC), a watchdog of hate groups. The online forum Reddit also hosts incel communities. Last November, it shut down its “Incels” message board, which had 40,000 members, after numerous posts promoted rape and violence against women.

Rodger killed six people near the campus of the University of California, Santa Barbara, in 2014, and wrote a manifesto in which he blamed women for his loneliness, claiming: “I’m the perfect guy and yet you throw yourselves at these obnoxious men instead of me, the supreme gentleman.”