According to Lawrence Wright in the book The Looming Tower: Al-Qaeda and the Road to 9/11, >A team of three French commandos from the Groupe d’Intervention de la Gendarmerie Nationale (GIGN) arrived in Mecca. Because of the prohibition against non-Muslims entering the holy city, they converted to Islam in a brief, formal ceremony. The commandos pumped gas into the underground chambers, but perhaps because the rooms were so bafflingly interconnected, the gas failed and the resistance continued. With casualties climbing, Saudi forces drilled holes into the courtyard and dropped grenades into the rooms below, indiscriminately killing many hostages but driving the remaining rebels into more open areas where they could be picked off by sharpshooters. More than two weeks after the assault began, the surviving rebels finally surrendered. https://en.wikipedia.org/wiki/Grand_Mosque_seizure