Democrats on the House Judiciary Committee have a new investigative emphasis this fall — they plan to focus on hush money payments arranged on behalf of women who had sexual encounters with President Trump, according to the Washington Post’s Rachael Bade and Tom Hamburger.

And one major unanswered question that they should push hard for answers on is: why, exactly, did the Justice Department’s investigation into this end earlier this year — when prosecutors had said that Trump directed these illegal payments?

There are several possibilities. Per NBC News’s Ken Dilanian, House Democrats suspect it was due to DOJ’s policy against indicting a sitting president. It’s also possible that prosecutors just thought their case wasn’t strong enough — or that there was interference from higher-ups.

The point is, we don’t know. In stark contrast to both the Hillary Clinton email probe and the Mueller investigation — for which prosecutors’ decision-making in not bringing further charges was publicly explained at length — what happened behind the scenes in the hush money investigation is a mystery.

It’s a mystery that House Democrats should try and solve. Rather than falling into the trap of just rehashing facts that have long been known about this scandal, Democrats should press hard to try and get answers on what happened in the Justice Department here.

What we know, and don’t know, about why the investigation was closed

Longtime Trump lawyer Michael Cohen pleaded guilty an August 2018 for violating campaign finance law in arranging hush money payments to Stormy Daniels and Karen McDougal, so they wouldn’t publicly discuss their sexual encounters with Trump before the 2016 election.

Soon afterward, the prosecutors who had brought the case — from the US Attorney’s Office for the Southern District of New York — announced they’d also struck a non-prosecution agreement with American Media Inc. — the National Enquirer’s parent company, which was involved in the McDougal payment.

Then in December, the SDNY prosecutors made clear their investigation wasn’t yet over. They wrote in a court filing that, in arranging the illegal payoffs, Cohen acted “at the direction of Individual-1” — Donald Trump. They were also scrutinizing the role of the Trump Organization.

But then, nothing else really happened. And in July 2019, the prosecutors involved told a judge that they’d “effectively concluded” their investigation, which was no longer “ongoing.” And that, it seemed, was that.

There is a spectrum of possibilities for what happened here:

Trump wasn’t charged because he was the president: The Department of Justice’s Office of Legal Counsel concluded back during the Clinton Administration that they could not indict a sitting president. They have stuck to this conclusion during the Trump Administration, and this holding played a major role in Robert Mueller’s decision-making during his probe (Mueller decided he should not come to any conclusion on whether Trump criminally obstructed justice, in part because charges could not be brought against him while he was in office).

How this policy might have played into SDNY’s hush money investigation has yet to be determined. Did SDNY back off because he’s the president and charging him would be pointless? Did they put the investigation in a drawer, to perhaps be revived later once Trump is no longer in office? Or did they outright conclude Trump committed a crime, but that they couldn’t charge him solely because he’s the president?

There was interference from higher-ups: Another question is whether prosecutors were told to back off by Justice Department higher-ups. There are some odd tidbits to the timeline here: CNN reported that prosecutors asked to interview Trump Organization executives about the payments in January but then, curiously, “never followed up.” Bill Barr was sworn in as attorney general in February. And the following month, Robert Khuzami — the prosecutor who oversaw the hush money probe — left government.

There’s been no actual evidence of any political interference or foul play prompting the investigation’s conclusion. No anonymous sources have leaked to reporters that something nefarious went on here, and the New York Times reported Khuzami has told colleagues his exit was “unrelated to any political pressure.” But in a Justice Department where the attorney general has gone out of his way to help Trump politically, it’s certainly worth a bit of oversight.

They closed the case for normal reasons: Prosecutors decline to bring new charges or pursue existing cases further all the time. and that could have been what happened here. Cohen is obviously a problematic witness, and prosecutors may have been hesitant to rely on his testimony. They could have concluded they lacked sufficient hard evidence against Trump (particularly on whether he knew he was violating the law). They also may have been hesitant to use what is at least a somewhat innovative interpretation of campaign finance law in a contested case. (Cohen pleaded guilty in part because they had other leverage on him, in tax and bank charges.)

Trump’s role as president means we need more transparency here — not less

The Justice Department naturally prefers not to talk about its behind-the-scenes investigative decisions. But, again, prosecutors have said in court documents that Donald Trump directed these illegal payments — payments to which Michael Cohen pleaded guilty to making.

There could be completely legitimate reasons not to move forward with the case. But DOJ’s insistence that they can’t charge the president, even if he did commit a crime, necessitates more oversight and more transparency here.

There are recent precedents for such transparency involving investigations of major politicians: the Hillary Clinton email probe and the Mueller report. Congressional Republicans aggressively demanded more details on why Clinton wasn’t charged, and James Comey’s FBI provided them. There was also successful pressure to get Barr to commit to releasing Mueller’s (mostly unredacted) report.

We now have an enormous amount of detail on the exact timeline of the Clinton probe and the behind-the-scenes decision making there — even though it involved a presidential candidate and not the president, and even though no charges were brought against anyone.

Comparatively, we know almost nothing about what went on behind the scenes in the Trump hush money probe, which did result in charges against Cohen, and has the added complication of the sitting president being immune from indictment. And DOJ has faced little heat over this.

Democrats’ oversight efforts all year have been hampered by their inability to bring much new information to the table. This is one area where there is a legitimate mystery that they should try and help answer. They may be stonewalled yet again — but it’s worth turning up the pressure on the Justice Department to try and get some answers.