Seattle Police Going Undercover at Sunday's Football Game Against the 49ers

warning tonight for rowdy fans going to football games as he head into this weekend. Word authorities are dressing up as fans and sitting there right beside you, to catch bad behavior. And where are they doing it? Here's abc's neal carlin skill now. Reporter: After years of fans being attacked at nfl games, sometimes violently, as seen on youtube, just for cheering the wrong team, seattle police are going after hooligans, right where they sit. Undercover officers will be dressed specifically in the visiting team's gear. This sunday, that's the san FRANCISCO 49ers. People coming to the game aren't going to know, hey, is this a fan that I can harass or give a hard time to or is that really a cop? Let them go at it! Reporter: Violenviolence, fan on fan, has become a frighteningly routine shade show at nfl games across the country. He hit me really hard. Really hard. It was devastating. I actually thought I was going to die, because I couldn't move. Reporter: It's the very reason other teams have launched similar stings, trying to crack down on violence so that fans can attend the game without worrying about being assaulted. Neal karlinsky, abc news, seattle.