A nonprofit clean energy group that ran electioneering ads urging voters to “thank Senator [Kay] Hagan” 23 days before the imperiled North Carolina Democrat faced a primary election failed to report its spending to the Federal Election Commission.

Federal rules require any ads that feature a “clearly identified candidate for federal office” less than 30 days before a primary election to be reported as “electioneering communications,” even if they stop short of calling on citizens to vote for or against a candidate. The regulations were approved to stop underhanded election ads from going totally unreported.

The “electioneering window” during which campaign ads in North Carolina’s hotly contested Senate race must be reported began April 6, one month before the May primary. TV documents show the ads, from Tennessee-based Southern Alliance for Clean Energy, began March 24 and ran through April 13.

There are few avenues for authorities to enforce the campaign disclosure rules, but records of political advertising maintained at local TV stations provide one plausible route. Beginning in August 2012, ABC, CBS, NBC and FOX stations in the top 50 markets have been uploading these documents to a Federal Communications Commission website. Though there’s no standard format for the ad buy documents, the public collection of documents provides a messy way to determine which TV advertisers need to be reporting their spending to the FEC. Sunlight also collects these documents at politicaladsleuth.com, where they are searchable across TV markets.

While electioneering ads airing less than 30 days before a primary must be reported, ads shown earlier than that need not be. Early advertising in Hagan’s reelection race was dominated by dark money advertisers who’ve spent millions running ads that go unreported to the FEC.

Jennifer Rennicks, a spokeswoman for the Southern Alliance for Clean Energy, said the ads were paid for by the group’s 501(c)3 arm and weren’t seen as political. While “social welfare” groups organized under section 501(c)4 of the tax code are allowed to get involved in elections, 501(c)3s are prohibited. View the Alliance’s latest filing to the IRS here.

After being sent links showing the ads apparently ran during the electioneering period, the the alliance released a statement saying they were looking into the matter. “Any ads that ran within the electioneering window were run in error,” the group said in a statement. “We are taking this very seriously and we’re examining what may have happened to determine our next course of action.”

The North Carolina ad appears to be a response to the barrage of attack ads, including these produced by Americans for Prosperity, against Hagan that began airing last year. “Who’s behind the attack ads on Kay Hagan?” asks the narrator. “Oil industry billionaires, that’s who.” The ad extols Hagan’s efforts to fight “corporate polluters” as footage of Hagan plays on the screen and her name appears in bold text.

A press release from the group described the campaign in North Carolina as “part of seven-figure national TV buy, digital outreach and 11 state organizing effort.” The ads aired in three North Carolina media markets, according to the press release, which didn’t give a dollar figure. The total cost of ads airing in the Charlotte was about $380,000 according to an estimate that appears in one ad document.

The campaign included both local and national groups, according to the press release. “Another component of the overall national campaign is six-figure digital ad campaign as well as a robust organizing plan for North Carolina and 10 other states including Maine, New Hampshire, Pennsylvania, Virginia, North Carolina, Ohio, Michigan, Colorado, Minnesota, Montana and Iowa … The activity in these states will occur throughout the remainder of the year and will be supported by groups including Environment America, Natural Resources Defense Council, Sierra Club, National Wildlife Federation, Earthjustice, Environmental Defense Fund and others.”