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This story was originally published by The Center for Public Integrity, a nonprofit, nonpartisan investigative news organization in Washington, D.C.

WASHINGTON — A last-minute infusion of cash is deluging the already unpredictable special Senate election in Alabama, and outside political groups backing both candidates — Republican Roy Moore and Democrat Doug Jones — are exploiting legal loopholes to hide their donors until after the Dec. 12 vote.

Some super PACs — groups that may accept and spend unlimited amounts of cash in elections — haven't publicly disclosed anything at all. Others have found creative ways around unveiling the backers fueling final pushes in the waning days of the race, when voters are bombarded with the most ads, mailers and phone calls.

"It's a failure of federal election law, and donors have reasons for wanting to stay hidden," said Brett Kappel, an election law specialist and partner at Akerman LLP. "We have an extremely controversial candidate that a lot of donors don't want to be associated with, and on the other side, people and companies may be worried about retribution if they publicly oppose Moore and he wins."

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Several of the super PACs, which operate independently of the candidates, are timing their spending to avoid triggering disclosure requirements. As of Thursday, 22 groups spent money boosting or blasting Alabama candidates since Oct. 1. Yet only half of them disclosed the names, addresses and contribution amounts of people who, through Nov. 22, gave them money.

Democratic senatorial candidate Doug Jones speaks at a news conference on Dec. 4, 2017, in Dolomite, Alabama. Brynn Anderson / AP

One newly-minted liberal group, Highway 31, aired millions of dollars of ads, despite having no money in its bank account. A conservative character scorched voters' inboxes with fiery rhetoric, yet the author's identity proves a challenge to uncover.

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The effect of the late outside spending is tough to predict in a race that has already seen more crazy twists than your average political thriller.

Moore seemed a lock to win Sen. Jeff Sessions' former seat. But a Nov. 9 Washington Post article was the first of several reports detailing accusations that Moore engaged in inappropriate sexual behavior with teenage girls, upending the race and boosting the chances of Jones, a former U.S. attorney. Moore has denied allegations of past non-consensual relationships with teenagers.

Political powerhouses who put millions of dollars behind Sessions' replacement Republican Sen. Luther Strange in the primary, including the Mitch McConnell-connected Senate Leadership Fund and the U.S. Chamber of Commerce, have so far declined to support Moore.

Others have dithered: The Republican National Committee and National Republican Senatorial Committee initially cut Moore off, but after the president personally endorsed Moore, the RNC jumped back in. The National Rifle Association, after staying quiet through early December, on Tuesday spent almost $55,000 mailing anti-Jones postcards to prospective Alabama voters.

Second-and-third-tier super PACs with scanty track records and little known about them have jumped in to fill the gap. Those groups, combined with a handful of bigger names such as the Human Rights Campaign and the National Association for Gun Rights PAC, have spent more than $5.7 million on ads, mailers and email blasts since Moore beat Strange in the Sept. 26 Republican runoff.

The $4.4 million boosting the Democratic candidate comes mainly from one mysterious super PAC and a group founded by Evan McMullin, who ran for president as an independent candidate in 2016. Moore, meanwhile, trails behind with the $1.3 million in support from a smattering of smaller entities.

Roy Moore speaks at a campaign rally in Fairhope, Alabama Dec. 5. Andrea Morales / for NBC News

Meanwhile, Jones' campaign committee raised more than $10 million from Oct. 1 to Nov. 22, while Moore's committee raised a comparatively paltry $1.8 million. Politico reported that Jones' campaign, as of late November, had spent about seven times more than the Moore campaign on TV ads. Campaigns, unlike super PACs, are subject to limitations: They can accept up to $2,700 from individuals and $5,000 from company PACs, and no money from corporations.

Jones' campaign refused to comment on outside groups' involvement in the race.

The mystery liberal behemoth

On Jones' side, a super PAC called Highway 31 spent nearly $4.2 million on mailers and online and television ads boosting Jones to date.

Who's funding this massive effort? That's not clear: Highway 31 is employing the legal, if rarely used, strategy of doing all its business on credit. That means it won't have to reveal donors — if it even has any, at the moment — until several weeks after Alabama voters have voted.

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The Center for Public Integrity is a nonprofit, nonpartisan investigative news organization in Washington, D.C.