The threat of a potential mass shooter at screenings of the upcoming Joker movie has prompted the U.S. Military to issue a warning to service members.

On Tuesday the U.S Army confirmed it had widely distributed its warning after the FBI uncovered extremist posts on social media by "incels" earlier in the month.

The pop culture website io9 is reporting that an email was sent to service members instructing them to "identify two escape routes" when entering theaters and if an active shooter entered the email advised, "Run if you can. If you're stuck, hide (also known as 'sheltering in place'), and stay quiet. If a shooter finds you, fight with whatever you can."

The FBI did not respond to io9's request for comment, but the Army said the warning was precautionary and that they were not aware of any specific plot.

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In their email alert they said that they suspected Joker screenings would be targeted as incels, "idolise the Joker character, the violent clown from the Batman series, admiring his depiction as a man who must pretend to be happy, but eventually fights back against bullies."

io9 describes incels as an, "online subgroup of self-professed 'involuntary celibate' men. Over time, some radicalised members of the incel community have formed an ideology that promotes violence.".

The active shooter who opened fire in an Aurora movie theater during a screening of the Batman movie The Dark Knight Rises in 2012 reportedly made statements to the police saying he, "was the Joker," becoming a hero to incels. The chief of police at that time however disputed the claim saying, "there is no evidence," that the shooter had said that. Another male shooter who opened fire near the University of California campus in 2014, killing six people, identified as incel.

The movie, which stars Joaquin Phoenix in the title role, has sparked controversy for the sympathetic manner in which the Joker is portrayed, framing him as an unloved "loser" who cracks and goes on a murderous rampage.

Relatives of the Aurora movie theater shooting have called on movie studio Warner Bros. to commit to gun control causes after being alarmed by the violence in the Joker trailer.

"When we learned that Warner Bros. was releasing a movie called 'Joker' that presents the character as a protagonist with a sympathetic origin story, it gave us pause," a letter they sent to Warner Bros. read. "We want to be clear that we support your right to free speech and free expression. But as anyone who has ever seen a comic book movie can tell you: with great power comes great responsibility."

Warner responded with a statement later Tuesday in which it said the company supported victims of gun violence and called on policymakers to enact bipartisan legislation to address America's gun epidemic.