A 19-year-old man was arrested after spending months planning an attack on a U.S. Army site in suburban Detroit, authorities said Wednesday. The man was planning a mass shooting at the Army’s Tank-Automotive & Armaments Command in Warren, commonly known as the Detroit Arsenal, on behalf of the Islamic State group, the U.S. Attorney's Office said. But he didn’t know that he had been scheming with two undercover FBI employees, who recorded audio and video images of their meetings with him, including handwritten diagrams of the site, referred by the Army as TACOM, authorities said.