Dallas police are investigating a convenience store robbery in which a group of teens attacked a clerk.The melee, which was caught on video, happened last month at the Exxon Tiger Mart at South Polk Street and Interstate 20, WFAA-TV (Channel 8) reports.A large crowd of young people entered the store about 10:15 p.m. and began throwing bottles at one another.http://crimeblog.dallasnews.com/archives/2011/09/video-shows-crowd-of-teens-bea.html