The FBI says it thwarted an attack on the Mall of America by a teenager after agents supplied him with an assault rifle. Authorities announced Wednesday that the 18-year-old had been under investigation for nearly two years, trading messages with a confidential informant. A federal complaint says Sheikhdoon Abdullahi Mohamud was arrested Tuesday in a public park after he paid $600 to an undercover FBI employee posing as a member of the Islamic State group for an AK-47 and 200 rounds of ammunition. It accuses Mohamud of repeatedly telling the informant that he wanted to kill non-Muslims. He was represented at his initial court appearance Wednesday by a public defender. The lawyer and members of his family declined to comment.