The United States attorney in Manhattan,Preet Bharara, said the jury unanimously found that Mr. Valle’s “detailed and specific plans to abduct women for the purpose of committing grotesque crimes were very real.”

“The Internet is a forum for the free exchange of ideas,” he continued, “but it does not confer immunity for plotting crimes and taking steps to carry out those crimes.”

The verdict came on the 12th day of Mr. Valle’s trial in Federal District Court, where he was also convicted of illegally gaining access to a law enforcement database that prosecutors said he had used to conduct research on potential victims. He faces up to one year in prison on that count.

After the verdict, several members of the six-man, six-woman jury said an early straw poll had shown a broad split on the main conspiracy charge: about four jurors believed Mr. Valle to be not guilty, another group of perhaps five saw him as guilty, and the rest were undecided, one juror recalled.

“I did not know which way I was going to go — I was leaning toward not guilty,” another juror recalled feeling as deliberations began. The jurors, who all spoke on the condition of anonymity, said they spent about half a day reading the judge’s instructions, and outlined the case chronologically with pieces of paper on a wall.

At one point, two jurors read Mr. Valle’s chats aloud, with one reading his words and another reading his conspirators’ words. His communications with one man, who used the screen identity Moody Blues, were pivotal, a juror recalled. Without those chats, the juror said, there might have been a not-guilty verdict.