It was minutes before the start of a concert by the popular rock band, Picnic, at a massive auditorium at Crocus City Hall, an entertainment complex on the northwestern outskirts of Moscow. Then, concertgoer Dave Primov heard the first popping sounds, but thought it was fireworks. As events unfolded, it became clear that it was the deadliest attack on Russian soil in years. The massacre turned into a manhunt, with authorities saying they detained four men in Russia's Bryansk reagion. President Vladimir Putin sought to tie Ukraine to the attack, but an affiliate of the Islamic State group claimed responsibility — which U.S. intelligence officials confirmed. Kyiv denied any involvement. Sunday was a day of mourning across Russia.