The 1990 law banned documenting animal abuse on factory farms.

Kansas cannot bar people from conducting undercover investigations on factory farms, the a federal court in Kansas ruled Wednesday.

For nearly 30 years — since 1990 — a Kansas state law made it illegal to take photographs or record video in a factory farm or slaughterhouse “with the intent to damage an enterprise conducted at the animal facility.”

The law was the earliest example of what are now called “ag-gag” laws, which criminalize undercover investigations, often by animal welfare groups, that reveal abuses on farms. Since Kansas’s law was enacted, half a dozen states have passed such laws — and more have considered it. Legislators have been forthright about their motives: They’re worried that evidence of what goes on on these farms will outrage Americans, so they want to ban it.