Where does it say POTUS can fire FBI director?

He can fire political appointees. I think regular hires would in most causes have to be fired for cause.

Non-political federal employees are protected by the Civil Service system. They can not be fired by the president.

Couldn’t he take the same actions that the park ranger’s normal superiors could, though (including, with appropriate cause, firing them)?

Yeah cause I remember some US civil servant held up the release of funds to Iran during the last days of the Carter Presidency (exchange of hostages) and Carter fired him.

Did Carter literally fire him, or was he fired by his boss for cause? It’s common to attribute to the president actions taken by his underlings. “Nixon bombed Cambodia” doesn’t mean that Nixon was piloting the aircraft dropping the bombs.

In addition to career civil servants, the political appointees in certain agencies are not subject to being fired by POTUS because the agencies were designed by Congress to be independent. Ravenman touched on this earlier. SCOTUS has used the shorthands “quasi-judicial” or “not purely executive” to describe the functions of those agencies, but Congress has guaranteed the terms of individuals in agencies with no judicial functions too (such as NASA). Typically they are called “independent agencies.”

Unless they serve longer, like Robert Mueller did.

There have been six FBI directors since Hoover (excluding acting and interim directors). Two were fired (Sessions and Comey) and one served 12 years, an extension that had to be approved by Congress.

The other three (Kelley, Webster, and Freeh) resigned well before their 10-year term-limit was reached.

The President CAN fire the FBI director, but there better be a damned good reason and C-SPAN discussions about it first. To say, "I don’t like you for petty political grudges, so I’m going to punish a whole country with a pox!

Whatever you do, don’t fire the non-Civil Service people in the WH travel office. That would be really, really bad.

The Senate had to approve his reappointment. It’s not clear what law allowed them to do so.

This law.

If it was entirely legal then there wouldn’t be any uproar about it having happened, and it wouldn’t be the object of an obstruction of justice assertion.

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The uproar over Comey’s firing is political, not criminal in nature. If there was obstruction of justice, it was (potentially) in Trump’s alleged requests that Comey discontinue the FBI investigation of Michael Flynn.

It was perfectly legal for Nixon to ask Elliot Richardson to fire Archibald Cox, and could have fired Richardson (if he hadn’t resigned first) as well. All perfectly legal, all political suicide.

There is some dispute over how independent “independent agencies” really are. They are clearly not part of the judicial or congressional branches, therefore they must be part of the executive, and the president, or so presidents assert, have untrammeled authority over the executive branch. So the EPA, for example, originally conceived of as an entity operating with congressional oversight but independent of the executive chain of command has gradually come to be more and more like a cabinet department.

I don’t even see how it is debatable that a president can fire appointees – it is inherently within his Executive power to do so. The post-Civil War statute was clearly unconstitutional.

At the same time, firing an FBI director with the intent to obstruct justice is itself criminal conduct and it is a failure to faithfully execute the laws of the land – both are easily impeachable offenses for which the President can be tried, convicted, and forcibly removed, or forced to resign.