From 2009 to 2011, the Restaurant Opportunities Center United spent just under $100,000 on lobbying expenses that it failed to disclose to the IRS, public tax documents show. The activist group conceded the omission in an IRS filing but offered no explanation for the failure to disclose.

ROC United is tax-exempt 501(c)(3) nonprofit group that agitates for higher wages and better conditions at restaurants.

The nonprofit's 2012 IRS filing states that it had lobbying expenditures of more than $15,000 in 2009, just under $30,000 in 2010, more than $52,000 in 2011 and more than $70,000 in 2012.

ROC United's filing states that it "failed to state lobbying expenses ... on its 2009-11 Forms 990." As a result, "this is flier's first form 990 that details lobbying expenditures." 990s are the IRS forms nonprofits must file. ROC United's was posted on the transparency website Guidestar.com.

The activist group makes a point of claiming in the IRS filing that its lobbying expenses were "well UNDER" (emphasis in original) the permitted legal limits for nonprofit groups like itself. The nonprofit did not respond to a request for comment from the Washington Examiner.

Last year, ROC Exposed, a conservative group, filed a complaint with the IRS that the liberal group was not disclosing its lobbying expenses.

"Even as ROC was securing taxpayer-funded grants and lobbying the union agenda, it was telling the IRS it was not lobbying. Now more than ever, we need IRS and congressional investigations of ROC, and an independent audit of ROC's books," said Mike Paranzino, spokesman for ROC Exposed, in a statement Tuesday.

According to its lobbying disclosure forms, ROC United currently has one lobbyist who registered with the federal government on Jan 21, 2013. The nonprofit focused primarily on lobbying members of Congress to raise the minimum wage.

ROC United has close ties to the food service workers union Unite Here. Paul Schwalb, the union's deputy director of food services, serves on the nonprofit's board. Big Labor favors a higher minimum wage because it makes nonunion labor less competitive.