Paul J. Manafort was in bed early one morning in July when federal agents bearing a search warrant picked the lock on his front door and raided his Virginia home. They took binders stuffed with documents and copied his computer files, looking for evidence that Mr. Manafort, President Trump’s former campaign chairman, set up secret offshore bank accounts. They even photographed the expensive suits in his closet.

The special counsel, Robert S. Mueller III, then followed the house search with a warning: His prosecutors told Mr. Manafort they planned to indict him, said two people close to the investigation…

“They are setting a tone. It’s important early on to strike terror in the hearts of people in Washington, or else you will be rolled,” said Solomon L. Wisenberg, who was deputy independent counsel in the investigation that led to the impeachment trial of President Bill Clinton in 1999. “You want people saying to themselves, ‘Man, I had better tell these guys the truth.’”