Who was paying for Fusion GPS' work during the GOP primary?

Prior to this week, we only knew -- thanks to CNN reporting -- that Democrats supportive of Clinton had funded the dossier, which was put together by a former British spy named Christopher Steele. Now we know that it was Clinton's campaign, not just Clinton backers, who were ponying up the money to fund the information collection on Trump and Russia.

There's still one big mystery here, however: Who was paying for Steele's work during the Republican primary? The Washington Post, who broke the story of the payment to Fusion GPS by Clinton and the DNC, offered a few clues in their story:

Clue #1: "Fusion GPS's research into Trump was funded by an unknown Republican client during the GOP primary."

Clue #2: "Fusion GPS's work researching Trump began during the Republican presidential primaries, when the GOP donor paid for the firm to investigate the real estate magnate's background."

That reporting corroborates this from The New York Times back in January:

"The story began in September 2015, when a wealthy Republican donor who strongly opposed Mr. Trump put up the money to hire a Washington research firm run by former journalists, Fusion GPS, to compile a dossier about the real estate magnate's past scandals and weaknesses, according to a person familiar with the effort."

And, we have some denials of involvement from some of the GOP candidates who ran against Trump in that primary.

On CNN's "New Day" on Friday, New Jersey Gov. Chris Christie said that his campaign had never paid Fusion GPS for any services. On Thursday, Florida Sen. Marco Rubio also denied any involvement in the funding of the dossier.

"As far as whether it was my campaign, it wasn't and I'll tell you why," Rubio said on CNN. "I was running for president. I was trying to win. If I had anything against Donald Trump that was relevant and credible and politically damaging, I would've used it. I didn't have it."

In an August Washington Examiner piece by Byron York, strategists for the super PACs of Jeb Bush and Ted Cruz also denied any involvement in funding the dossier -- or even knowing it existed.

Assuming -- and some Republicans don't -- that this GOP donor actually exists, it's not terribly surprising that none of the major campaigns seem willing to take credit or blame for the dossier.

Because this was a project funded by a GOP donor, there's no reason to think that donor was acting at the behest of the campaign. He or she could have simply given the money to Fusion GPS and then doled out -- or planned to dole out -- whatever good nuggets Steele found to the most viable anti-Trump candidate left in the field.

The only possible link I could see between this GOP donor and a campaign is if he or she made contributions to a candidate or a candidate's super PAC -- in addition to the money spent on the Fusion GPS research. Plus, while it was a Republican who brought Fusion GPS into the mix, we don't know, and there is no evidence yet, that Fusion had even retained Steele while it was being paid by the Republican. There's plausible deniability everywhere.

The simple fact is this: The identity of the Republican donor who began this whole dossier odyssey is now the last major mystery surrounding a document that has roiled Trump's first year in office.

Will we ever find out who the mysterious donor is? I say yes because, well, my experience in politics (and life) is that everything comes out in the end.