A video circulating on social media sites, including Twitter, on May 18 purports to show several armed men pledging allegiance to the Islamic State in an unknown location in a cave in Tanzania. The men in the video go on to call for other East Africans to join their cause. Much about the video remains unverified, including the time and place it was shot. But Tanzanian Defense Minister Hussein Ali Mwinyi has warned of growing numbers of Tanzanians joining the Islamic State and Somalia's al Shabaab . Additionally, tension between Muslims and Christians in Tanzania is nothing new, particularly on the semi-autonomous island of Zanzibar, and religious sites of both faiths have been attacked in the past.