Over nearly two years, special counsel Robert S. Mueller III has operated like a prosecutorial submarine, abruptly surfacing without warning to release indictments or announce guilty pleas and then plunging back down to continue the Russia investigation.

He is ubiquitous and yet invisible — the last known photograph of Mueller was snapped last summer. He has given no interviews, a reticence that has only stoked anticipation for his final report, which appears imminent.