he fdls the fdls has been called one the most dangerous polygamist cults in the nation. their leader warren jeffs behind bars for sexually assaulting two young girls he said were his spiritual wives. >> flu two former followers are risking their lives to help others escape. we are "all up nightline" with cecilia vega. >> reporter: we are getting close now, right? >> we are just entering. >> reporter: this is the hidden world of the flds, one of the largest polygamist groups in the united states and the kingdom of their leader, the now jailed warren jeffs. it doesn't take long to get the message, we are not welcome here. did they do that because of us? >> yeah, yeah. >> reporter: that's because we are riding with two of his biggest enemies, his own flesh and blood. >> my name is mathew wall