No, they aren't after people who litter. The National Park Service's Investigative Services Branch (ISB) of 33 special agents handle the rapes, murders, assaults, robberies, poaching, drug smuggling, and other big crimes that occur on the 85 million acres of United States parks, historical sites, monuments, and other NPS turf. From Outside:
ISB agents are a strange breed. They require a high tolerance for time alone in the backcountry—but because solving crimes typically comes down to getting information from people, they also need social skills. "I look for people who can talk to anybody," Sullivan told me. Each of the half-dozen agents in the office was drawn to the job for different reasons. Kristy McGee, a petite blonde wearing cowboy boots, specialized in violent crime. "I had a very chaotic childhood. I was exposed to a lot of adult-natured things—drugs, abuse," she told me. "I found a place where I can use that to relate to people."
Steve Kim, who has salt-and-pepper hair and a degree in wildlife ecology, told me about how he had spent the summer of 1995 living the life of a dirtbag climber, when Yosemite put out a call asking climbers to help with a death investigation. While rappelling off the east ledges of El Capitan, looking for clues, Kim discovered that ISB work suited him—"It's probably my obsessive-compulsive tendencies"—and never looked back.
Cullen Tucker, the office's youngest agent at age 30, was born into the business; his dad is a former deputy chief ranger at Yosemite, and his mom was one of the park's first female investigators. Thirty years ago, the case of the dead girl in the meadow (the one whose murder was claimed by Henry Lee Lucas) had been assigned to Cullen's mom, Kim Tucker. With very little to go on, she hadn't been able to identify the body. "I've never given up thinking about it," she told me. Sullivan recently reassigned the cold case to her son.