Special counsel Robert Mueller just explained why he couldn't charge President Trump with a crime.

Mueller reiterated the finding in his report, saying if his office "had confidence that the President clearly did not commit a crime, we would have said so."

"We did not, however, make a determination as to whether the President did commit a crime," he said.

Mueller then explained, citing Department of Justice policy, that a President "cannot be charged with a federal crime while he is in office. That is unconstitutional."

"Even if the charge is kept under seal and hidden from public view, that, too, is prohibited," Mueller said. The special counsel's office is part of the Department of Justice, he said, and is therefore bound by its policies.

"Charging the President with a crime was therefore not an option we could consider," he said.