What they were doing, new interviews show, was going back to square one, poring over the reams of evidence that had already been collected by his predecessor.For a time, their efforts were haphazard as they examined a wide range of Mr. Trump’s business practices, including whether he had lied about his net worth, which was the focus of the investigation when Mr. Bragg had declined to seek an indictment. But by July, Mr. Bragg had decided to assign several additional prosecutors to pursue one particular strand that struck him as promising: a hush-money payment made on Mr. Trump’s behalf to a porn star during the final days of the 2016 presidential campaign.