Hope Hicks may hold the keys to Mueller's Russia puzzle

As a senior White House adviser and now as communications director, she’s been in the room for moments critical to Mueller’s probe, which has grown to include the president’s response to the Russia investigation itself.

Hicks’ history with Trump makes her one of the more useful witnesses for Mueller as he looks for insights into the president’s habits and moods. She also is one of the few people well positioned to recount the president’s reactions at various moments as the Russia scandal has sidetracked his presidency — including the Mueller appointment itself.

Mueller’s decision to request an interview with Hicks — who hasn’t been named in any criminal wrongdoing — also indicates he’s reached a critical point in the overall investigation, according to former prosecutors and veterans of past White House investigations. Typically, conversations with such senior level aides are saved for near the end of a probe.