MILWAUKEE — Authorities often learn about it from landowners who hear shooting at night and find deer carcasses in their fields the next day.

One case came to light when a bloody deer heart was discovered in a girl's high school locker. And some incidents are solved when conservation wardens catch the criminals red-handed shooting from roads with the help of spotlights and headlights, then leaving the wounded and dying animals behind.

Conservation wardens call it "thrill killing" of animals, and it appears to be a growing problem throughout Wisconsin, said Chief Warden Randy Stark.

Several dozen cases have been confirmed in the past few years, mostly involving young men in their late teens and early 20s. Officials say suspects have been killing wild animals by shooting them with firearms and arrows, running them down with vehicles and clubbing them with baseball bats and homemade weapons, such as sharpened sticks.

When asked why, those that can muster a reason don't say it's because they want the venison or antlers — it's because they're bored.

"This is simply a criminal act. It has nothing to do with hunting," said Department of Natural Resources warden Rick Rosen, who is stationed in St. Croix County. "It has nothing to do with sportsmanship at all. As far as why, it's just like retail theft. That's the best analogy. It's the thrill with getting away with something and not getting caught."

It's different from poaching because poachers, even though they're flouting Wisconsin's laws, are not leaving the animals to waste.

And there's a reason why it's illegal to shine lights and shoot guns at night for much of the year in Wisconsin — it's dangerous, and not just for the animals. The shooters don't know what might be beyond their target.

Jeremy Peery, a conservation warden in Rusk County, arrested three high school students several years ago who spent their summer driving around at night and shooting animals.

They "shot sandhill cranes, they shot turkeys, they even shot at some sturgeons that were spawning and porpoising up near a dam. Just for the kicks of doing it," said Peery, who added that just about every warden in the state has heard of or investigated a similar case.

The three paid fines ranging from $2,000 to $4,000, served jail terms and lost their hunting and fishing privileges for years.

In neighboring Chippewa County, Peery said, another warden handled a case in which high school students spent their evenings traveling around the northern edge of the county, shining lights at deer and shooting the vulnerable animals paralyzed by the bright lights. They cut out the heart of one of the deer and put it in the locker of the ex-girlfriend of one of the boys.

Ted Dremel, a DNR warden in Waupaca County, has investigated a couple of thrill killing cases, including one in 2006 that's now winding its way through court. Three young men admitted to killing or wounding 48 deer in the Iola area. Some deer were killed when the men shone a pickup truck's headlights into a field and stood on the road blasting away. Most of the dead deer were left behind.

"They truly give hunters a bad name. I think it's important to make a huge distinction between hunters and this type of activity," Dremel said.