If the special counsel, Robert S. Mueller III, has proved anything in his 18-month-long investigation — besides how intensely Russia meddled in an American presidential election — it is that Mr. Trump surrounded himself throughout 2016 and early 2017 with people to whom lying seemed to be second nature.

They lied to federal authorities even when they had lawyers advising them, even when the risk of getting caught was high and even when the consequences for them were dire.

Even more Trump associates are under investigation for the same offense. They are part of a group of people surrounding Mr. Trump — including some White House and cabinet officials — who contribute to a culture of bending, if not outright breaking, the truth, and whose leading exemplar is Mr. Trump himself.

Mr. Trump looks for people who share his disregard for the truth and are willing to parrot him, “even if it’s a lie, even if they know it’s a lie, and even if he said the opposite the day before,” said Gwenda Blair, a Trump biographer. They must be “loyal to what he is saying right now,” she said, or he sees them as “a traitor.”