Washington (CNN) Robert Mueller ended his two-year stint as special counsel with a bang disguised as a whimper: In a 10-minute statement announcing his resignation and the closure of the special counsel's office, the former FBI director sent a very clear message to anyone listening: I didn't charge Donald Trump with obstruction because I couldn't.

"The Special Counsel's Office is part of the Department of Justice and, by regulation, it was bound by that Department policy," said Mueller, referencing an Office of Legal Counsel ruling that a siting president cannot be indicted. "Charging the President with a crime was therefore not an option we could consider."

And just in case you missed what Mueller was driving at with that quote, he was even more explicit later in his remarks. "The [OLC] opinion says that the Constitution requires a process other than the criminal justice system to formally accuse a sitting president of wrongdoing," Mueller added.

So, to summarize:

Mueller says the special counsel's hands were tied by the OLC opinion when it came to charging Trump with obstructing the Russia probe. Mueller notes that "the Constitution requires a process other than the criminal justice system to formally accuse a sitting president of wrongdoing."

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