The Justice Department’s decision to appoint former FBI Director Robert Mueller as a special counsel charged with investigating possible collusion between the Trump campaign and Russia is a win for Democrats, a new blow to a reeling White House, and a clear sign that the scandal that has engulfed the administration will only accelerate in the weeks and months ahead.

It wasn't just the decision to appoint a special counsel that raised eyebrows. It was also the person put into the job: Mueller, who spent his career in the FBI and who takes the post just weeks after Trump startled lawmakers and officials from both parties by abruptly firing FBI Director James Comey (who is also, incidentally, Mueller’s close friend).

In the aftermath of the firing, congressional Democrats began calling for a special counsel or special prosecutor to take over the investigation because of fears that the Justice Department wouldn't be able or willing to properly investigate Trump and his current and former aides. But few seemed to think that would actually happen.

That's why the announcement by Deputy Attorney General Rod Rosenstein came as a surprise to so many people — including the White House, which was reportedly given just 30 minutes’ warning before the announcement went public.

If Rosenstein’s name sounds familiar, it’s because the administration initially claimed that Trump only decided to fire Comey after Rosenstein sent a letter to the White House recommending that he do so. This turned out to have been a lie — Trump later admitted that he’d already decided to fire Comey long before Rosenstein’s letter.

So when Rosenstein announced that he had appointed a special counsel, it had the feel of a career lawyer with a reputation for probity and nonpartisanship striking a blow at a White House that had thrown him under the bus by trying to make him the scapegoat for the Comey firing.

“My decision is not a finding that crimes have been committed or that any prosecution is warranted,” Rosenstein said in a DOJ press release. “I have made no such determination. What I have determined is that based upon the unique circumstances, the public interest requires me to place this investigation under the authority of a person who exercises a degree of independence from the normal chain of command.”

There's a lot to unpack there, including the fact that Mueller won't have the powers many people assume. He also won't be entirely independent. But he'll have a lot of power all the same — and is probably almost literally the last person Trump would want in the job.

A special counsel is weaker than an independent special prosecutor

Special prosecutors and special counsels are both appointed to remove conflicts of interest in federal criminal investigations, if the situation would otherwise lead to officials investigating their own superiors. They are supposed to be independent, usually either judges or highly respected attorneys.

But while the two terms are often used interchangeably in the media — and, more confusingly, by members of Congress who should know better — they’re not quite the same thing.

In 1978, in the wake of the Watergate scandal, Congress passed the Ethics in Government Act, creating something called a “special prosecutor” (also called an “independent counsel”) who would be independent from the supervision and control of the president. The role was to investigate and prosecute — in a court of law — possible violations of federal law by high-level officials.

The 1978 law also created a special three-judge panel of the US Court of Appeals for the District of Columbia Circuit, appointed by the chief justice of the Supreme Court, that would be responsible for appointing and overseeing a special prosecutor when formally asked to do so by the attorney general.

Under that law, only the attorney general had the authority to request that the panel appoint a special prosecutor. Majorities on either the House or Senate Judiciary Committee could formally ask the attorney general to file a request, but — because of the separation of powers enshrined in the Constitution — the attorney general could still technically say no. They would merely have to tell Congress why they were refusing to do it.

The law creating special prosecutors expired in 1999, when Congress chose not to renew it. That meant that the whole mechanism for establishing an independent special prosecutor went away.

So the Justice Department decided instead to use its own powers to create something similar within the department itself: a “special counsel.”

This person is appointed directly by the attorney general (not an independent three-judge panel, like before) and — this is the important part — is thus directly answerable to the attorney general. This person can pursue criminal indictments in grand jury investigations, but the attorney general can fire them at any time and overrule any decisions they make when it comes to investigating or prosecuting the case.

In other words, the special counsel is far, far less independent than the earlier version was.

Still, it's a powerful post. Mueller will be able to issue subpoenas, forcing people to testify and turn over documents. Whether he chooses to do so, of course, remains to be seen.

So that’s what we have today. In this case, Attorney General Jeff Sessions had already formally recused himself from participating in any Trump-Russia investigations, so the next person in line, Rosenstein, was the person with the authority to appoint a special counsel.

What does this mean for the investigation going forward?

“Special Counsel Mueller will have all appropriate resources to conduct a thorough and complete investigation,” Rosenstein said in the DOJ announcement.

So what, exactly, is the FBI chief investigating?

The order specifically gives Muller the legal authority to investigate “any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump” as well as any crimes that may take place during the investigation, including things like perjury, obstruction of justice, and witness intimidation.

And if Mueller finds enough evidence, he can seek an indictment against the suspect or suspects from a federal grand jury. If the grand jury chooses to indict, the case would then potentially go to a federal criminal trial.

However, there are some caveats.

Peter Zeidenberg, who served as the assistant special counsel in the investigation of former White House aide Scooter Libby in 2005, noted in a recent Washington Post op-ed that special counsels “are not journalists, and their job is not to inform the public of the results of their investigations.”

“Rather, their mission is to gather all of the relevant facts and determine whether a crime was committed and, if so, whether it can be proved in court beyond a reasonable doubt,” Zeidenberg wrote. “Their work, when done properly, is done in secret."

Which means that Mueller could very well find a bunch of fishy or wildly inappropriate stuff that Trump or his campaign team did, but if none of those things were actually illegal — or if Mueller doesn’t think he has enough evidence to bring the case to federal court — the public might never hear about any of the stuff he found.

(There are still the separate and ongoing House and Senate probes into Trump’s Russia ties, though, and the members of Congress involved in those probes could presumably subpoena any documents the special counsel investigation collected and choose to reveal those to the public if they wanted to.)

And, of course, Mueller can still be dismissed at any time by Deputy Attorney General Rosenstein, who can in turn be dismissed by Trump.