Intent on finding out more about Mr. Assange’s dealings with Russian intelligence, the C.I.A. began last year to conduct traditional espionage against the organization, according to American officials. At the same time, federal law enforcement officials were reconsidering Mr. Assange’s designation as a journalist and debating whether to charge him with a crime.

Mr. Pompeo and former Attorney General Jeff Sessions unleashed an aggressive campaign against Mr. Assange, reversing an Obama-era view of WikiLeaks as a journalistic entity. For more than a year, the nation’s spies and investigators sought to learn about Mr. Assange and his ties to Russia as senior administration officials came to believe he was in league with Moscow…

A prosecution of Mr. Assange could pit the interests of the administration against Mr. Trump’s. Mr. Assange could help answer the central question of the investigation by the special counsel, Robert S. Mueller III: whether any Trump associates conspired with Russia to interfere in the presidential race. If the case against Mr. Assange includes charges that he acted as an agent of a foreign power, anyone who knowingly cooperated with him could be investigated as a co-conspirator, former senior law-enforcement officials said.