The trial’s opening arguments underscored that theme. A federal prosecutor, Randall W. Jackson, told jurors that the officer had been plotting real crimes to kill actual victims, while Officer Valle’s lawyer, Julia L. Gatto, contended that he had merely been living out deviant fantasies in Internet chat rooms, with no intention of carrying them out.

One outside expert, Joseph V. DeMarco, an Internet lawyer and former head of the cybercrime unit in the United States attorney’s office in Manhattan, said in a recent interview that beyond its sensationalism, the Valle case highlighted the fact that there were “dark corners” of the Internet “where a whole range of illegal and immoral conduct takes place, and the general public has only a vague and fleeting knowledge that these places exist.”

Image Gilberto Valle

He noted that the Internet, as a medium of expression and communication, also made it possible for people with interests as benign as stamp collecting or as grisly as cannibalism to find and validate one another in community forums.

“If you were someone mildly interested in cannibalism 30 years ago, it was really hard to find someone in real space to find common cause with,” Mr. DeMarco noted. “Whereas online, it’s much easier to find those people, and I think when you have these communities forming, validating each other, encouraging each other, it’s not far-fetched to think that some people in that community who otherwise might not be pushed beyond certain lines might be.”

It was clear that the prosecutors, in their opening statement and through Ms. Mangan-Valle’s testimony, were seeking to bring as much realism to the courtroom as possible.

Ms. Mangan-Valle, who had taught with Teach for America and went on to become a teacher in East Harlem and in the Bronx, indicated that she had been so afraid for herself and their infant daughter that she flew to stay with her parents, who live in Nevada. She said she contacted the Federal Bureau of Investigation, gave a statement and granted the bureau access to her laptop and another computer in their home.