Who spray paints the graffiti along the highways in the inner city?

I have always wondered this. I've worked in Detroit for nearly my entire career, and they never clean it up, but someone always comes along and freshens it up or covers it and paints something new. This spring we have all kinds of artwork with interesting and/or profane four-letter words, some I've never heard of before, decorating our freeways. Is there a community of artists out there? Do they ever catch and arrest anyone for this?