“He built it. He grew it. He operated it from top to bottom until the very end,” Mr. Turner said.

The prosecutor’s comments came as the government and the defense clashed in closing arguments during the fourth week of Mr. Ulbricht’s trial in Federal District Court. The jury is expected to begin deliberations on Wednesday after instructions from Judge Katherine B. Forrest.

Mr. Turner said Mr. Ulbricht’s “criminal turf” had been a “dark corner of the Internet,” and the website “made it easier for drug dealers to get users hooked, users from all over the world.”

“His conduct was brazenly illegal,” Mr. Turner said. “He knew perfectly well what he was doing the whole time.”

Mr. Ulbricht’s lawyer, Joshua L. Dratel, had earlier told the jury that his client created Silk Road as a kind of “economic experiment,” and after a few months handed it off to others — including the real Dread Pirate Roberts — before being lured back and made a “fall guy.” Mr. Ulbricht was arrested by the Federal Bureau of Investigation in San Francisco in October 2013.

On Tuesday, Mr. Dratel pressed the contention that his client had been framed. He suggested the actual Dread Pirate Roberts and confederates had planted or edited the incriminating chats and other evidence that the government had used to tie Mr. Ulbricht to the website’s activities. “The Internet is not what it seems,” Mr. Dratel said, adding, “You can create an entire fiction.”