NEW: The FBI has thwarted a terror attack on the Mall of America in Minnesota involving an ISIS sympathizer named Sheikhdoon Abdullahi Mohamud. The 18-year-old male was arrested and charged for allegedly planning a mass casualty attack. Mohamud was reportedly under investigation for multiple years and traded messages with a confidential informant. Mohamud was arrested on Tuesday in a public park when he tried to pay $600 to an undercover FBI employee for an AK-47 and 200 rounds of ammunition. In text messages, he allegedly confirmed his intention to carry out an attack against non-Muslims. Mohamud had previously told the informant that he planned on going to Somalia to join the Islamic State. When he came to the conclusion that he didn't have the money to make it overseas, he allegedly decided to commit to martyrdom and commit mass violence at home in Minnesota. "I am 100% committed to the cause of dying for the sake of Allah only," he said in one message. He planned to target the Hmong Arts Celebration at the Mall of America on Oct. 24 because "it will be a lot of people."