The report was the first in-depth federal look at the prevalence of stalking, which is a crime in all 50 states. While many people tend to associate stalking with the pursuit of stars like Uma Thurman and David Letterman, researchers found that 3.4 million people were subjected to stalking, defined as a course of conduct that would cause a reasonable person to feel fear. Women were more often the victims than men. And 11 percent, about 374,000 people, had been stalked for five or more years.

And then there were those like Cameron Wallace of New Franklin, Ohio, who endured the terrifying experience far longer. Ms. Wallace, now 28, was in her sophomore year of high school in 1996 when she sat next to Ryan Clutter in art class. Although they never dated or were even friends, he began turning up just about everywhere she went.

For the next 11 years, he appeared at her house or at the mall, sat behind her at the movies, sent demands by e-mail and threatened her life. He described how he would kill her: “He was going to gut me,” she said in an interview this month, still tearful.

Yet, she said, the police told her that it was hard to “connect all his actions” and that he had denied them. “They could not act until he did something more serious,” Ms. Wallace said.

Three-quarters of victims know their stalker, whether it is a current or former friend, roommate or neighbor, this study and others have found. “Often stalkers want to make their victims fearful,” said Eugene A. Rugala, a former F.B.I. profiler who advises on workplace threats. “They are thinking, ‘How dare you do this to me? I’m going to make you pay.’ But others feel it could be a way of getting back into the relationship.”