On 20 July, news broke that Hitler and several officers of the Supreme Command of the Armed Forces had become victims of an explosion in the German military's headquarters on the Eastern Front, the Wolfsschanze (Wolf's Lair), near Rastenburg, East Prussia (now Kętrzyn, Poland). Fromm quickly concluded that it was Stauffenberg and the plotters who were behind the explosion, and when he attempted to arrest them, Fromm was quickly overwhelmed and confined to a prison cell in the Bendlerblock, the Berlin headquarters of the Replacement Army, among other branches of the German military, after he had refused to join the plotters in Operation Valkyrie (they had forged Fromm's signature to begin the operation).