Mueller’s decision to request an interview with Hope Hicks — who hasn’t been named in any criminal wrongdoing — may indicate 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.

“Anytime you can get someone who is the right-hand person or who’s been around the primary target of an investigation, under oath, answering detailed questions, means you’ve progressed very far along in the investigation,” said Adam Goldberg, a former Clinton White House lawyer.

Hicks’ long history with Trump makes her one of the more useful witnesses for Mueller.