MONTGOMERY, Ala. — A mysterious super PAC that spent millions of dollars backing Democrat Doug Jones in Alabama's Senate race was heavily funded by the Democratic Senate Majority PAC.

Chris Hayden, spokesman for the Senate Majority PAC, said Tuesday that the group was the primary backer of the PAC called Highway 31, which spent more than $4 million on hard-hitting advertising and mailings to help defeat Republican Roy Moore. Because of reporting and payment schedules, Highway 31 didn't disclose its donors during the campaign despite its heavy spending.

"Yes, SMP was the contributor to Highway 31. There were a few small donations when Highway 31 became public, but it was predominantly funded by SMP," Hayden wrote in an email.

Senate Majority said it is solely dedicated to building a Democratic majority in the U.S. Senate, where Republicans will hold a slim 51-49 lead in 2018. Jones is the first Democrat elected to the U.S. Senate from Alabama in a quarter of a century.

Hayden said the Senate Majority PAC spent more than $6 million in Alabama. That included: $2 million on television and radio; $2 million on a voter turnout operation in partnership with BlackPAC; $1.5 million on digital advertising in partnership with Priorities USA; and $700,000 on direct mailings. Priorities USA is another Democratic super PAC.

Throughout the Alabama race, Jones had performed something of a balancing act — taking help from some national Democrats — such as former Vice President Joe Biden and New Jersey Sen. Cory Booker who campaigned with him — while also stressing that he would be an independent voice in the U.S. Senate.

Highway 31, named after a major route crossing much of the state, emerged as a major player in the election. Many of its spots and mailers focused on accusations against Moore of sexual misconduct involving teenage girls decades ago. He denied the misconduct accusations.

Because it had not reported donors, Moore's campaign earlier this month criticized the group as "shadowy" and said it was purposely keeping the public in the dark about who was providing it money.

Highway 31 spokesman Adam Muhlendorf has said the group, which organized in the runup to the election, followed "every appropriate rule and regulation." It is expected to file its yearly financial report at the end of January.

"We formed Highway 31 to make sure every Alabamian knew Doug's background and to help him get out the vote. We couldn't be more proud of this campaign and look forward to him promoting Alabama values in the U.S. Senate." Muhlendorf wrote in an email.

On election night, J.B. Poersch, president of Senate Majority PAC, called Jones' win, "a great victory for the state of Alabama and a harbinger of difficult times to come for Republican Senate candidates throughout the country."