A mysterious super PAC has spent $2.3 million in the final stretch of the campaign opposing GOP Sen. Ted Cruz, but Texas voters will not know who was trying to influence their decisions through the group’s last-minute ads until weeks after they have cast their ballots.

The group, Texas Forever, is using a tactic that has become increasingly common this election cycle: a pop-up super PAC being created or running ads in a way that circumvents federal election deadlines that trigger donor disclosure. The groups then pour money into a competitive race with a barrage of ads, with no insight as to who is financing their last-minute endeavor until weeks after voters have made their decision.

Donors to super PACs are disclosed to the public in regular federal filings. But these super PACs, which keep their donors disclosed until after the election, have become more prevalent, according to a Center for Public Integrity analysis.

Similar previous efforts this cycle have been tied to Senate Majority PAC, the main super PAC aligned with Senate Democrats. SMP was the primary donor to Red and Gold, a super PAC that was active in the Arizona Senate primaries attacking Republicans, and to Highway 31, which spent heavily to help Democrat Doug Jones win the Alabama Senate race in 2017. In both cases, the super PACs were structured so that donors would not be disclosed until after the election.

Texas Forever and SMP did not respond to a request for comment.

Another such group that popped up just before Election Day is Scottsdale-based Restore Our Healthcare Inc., which has spent $46,250 running ads in support of Republican Rep. Martha McSally, running in a tight race for the Senate seat in Arizona against Rep. Kyrsten Sinema (D). The group launched Oct. 23 and will not be disclosing its donors until after the election.

The group’s treasurer is listed in federal filings as Randy Pullen, former Arizona GOP chairman. Pullen did not respond to a request for comment.

Another group appeared to be a new super PAC that spent about $9,900 in the final stretch to support Sen. Jon Tester of Montana, home to one of the most competitive reelection Senate races this fall. Forward Montana, which is based in Missoula, Mont., is a political nonprofit that does not disclose its donors, and the group said a staffer erroneously filed it as a super PAC.

Forward Montana, a youth civic engagement organization created in 2004, said it decided to participate in political activity this year by endorsing candidates in federal races because it determined “that this year’s election is too important to sit on the sidelines when we know the candidates offer starkly different visions for our future.”

The group voluntarily disclosed the source of the $9,900 donation: Diana Blank, a longtime donor from Bozeman, Mont. “We are happy to disclose her $10,000 in support” of the group’s work, said Rachel Huff-Doria, the group’s executive director.