MINNEAPOLIS, Oct. 9 -- The rifle that Sheikhdoon Abdullahi Mohamud carried out of a car in a Richfield parking lot on Tuesday could not fire. The FBI had made sure of that before an undercover employee sold it to him. Minutes later, agents arrested the 18-year-old, who prosecutors say planned a mass shooting at the Mall of America later this month.

Mohamud, of Richfield, Minnesota, was charged by complaint with one count of knowingly receiving a firearm and ammunition in order to commit a federal crime of terrorism, the Justice Department said. He made his first court appearance on Wednesday and was ordered held until a detention hearing on October 19. He has been appointed a federal defender.

According to the complaint, Mohamud began e...