WASHINGTON, Sept 22 (Reuters) - U.S. President Donald Trump signed an executive order on Monday calling the antifa movement a “terrorist organization,” the White House said, after promising actions targeting left-wing groups following Charlie Kirk’s assassination.

Kirk, a prominent conservative activist with close ties to Trump, was assassinated on September 10 while speaking on a college campus in Utah. A 22-year-old technical college student has been charged with Kirk’s murder.

Investigators are still looking for a motive and have not said the suspect operated in concert with any groups. But the Trump administration has used the killing as a pretext to revive years-old plans to target left-wing groups they regard as being hostile to conservative views.

Antifa, short for anti-fascist, is a “decentralized, leaderless movement composed of loose collections of groups, networks and individuals,” according to the Anti-Defamation League, which tracks extremists.

“While some extreme actors who claim to be affiliated with antifa do engage in violence or vandalism at rallies and events, this is not the norm,” it says on its website.