MINNEAPOLIS (AP) — The FBI thwarted an attack on the Mall of America by a teenager after agents supplied him with an assault rifle, authorities announced Wednesday.

The 18-year-old had been under investigation for nearly two years, trading messages with a confidential informant, before agents sold him the rifle and ammunition, they said.

“This was a radicalized young man who needed to be watched,” U.S. Attorney Daniel Rosen said as he praised the FBI investigation. Agents “were able to detect a threat, and disrupt it, before the defendant carried out his intention to kill, in his words, maybe 30 to 60 people, as we allege in far greater detail in the complaint.”

Sheikhdoon Abdullahi Mohamud, 18, was arrested Tuesday in a public park in Richfield, several miles northwest of the Mall of America, after he paid $600 to an undercover FBI employee posing as a member of ISIS for an AK-47 and 200 rounds of ammunition, according to the criminal complaint.

Sheikhdoon Abdullahi Mohamud, 18, was arrested Tuesday in a public park after he paid $600 to an undercover FBI employee posing as a member of ISIS for an AK-47 and 200 rounds of ammunition, according to the criminal complaint.

In text messages with a confidential FBI source, Mohamud repeatedly affirmed his intent to carry out an attack to kill non-Muslims, the complaint said. It was not immediately known if the teen had an attorney who might speak on his behalf.