Federal authorities say they narrowly prevented a mass casualty attack at the Mall of America after an undercover FBI operation led to the arrest of 18-year-old Sheikhdoon Abdullahi Mohamud. U.S. Attorney for Minnesota Dan Rosen announced Wednesday that Mohamud had been under FBI investigation for nearly two years, during which he allegedly exchanged messages with a confidential source he believed to be connected to ISIS and repeatedly expressed his intent to carry out an attack on behalf of the group. According to a newly unsealed federal complaint, Mohamud allegedly settled on October 24 as the date for the attack, allegedly targeting a Hmong Arts Celebration event at the mall with the stated goal of killing dozens of people.

The case came to a head on October 6, when Mohamud allegedly paid 600 dollars to an undercover FBI employee posing as an ISIS associate in exchange for an AK-47 style rifle and roughly 200 rounds of ammunition, according to court documents. Agents arrested him on the spot in a Richfield, Minnesota public park once he took possession of the weapon, ending what Rosen described as a carefully managed, two-year effort to monitor and ultimately stop the threat before it could materialize. Mohamud has been federally charged with knowingly receiving a firearm and ammunition for use in a federal crime of terrorism, among other counts, in a case now before the U.S. District Court for the District of Minnesota, and officials say the operation underscores that the threat of ISIS-inspired violence in the United States remains active.