An investigation by nonprofit journalism group ProPublica has uncovered the identity of one of the secret super PACs funding advertisements attacking U.S. Sen. Sherrod Brown (D-OH) and promoting his challenger, Ohio state treasurer Josh Mandel. The group is the Government Integrity Fund and is headed by Columbus lobbyist Tom Norris. Norris’ lobbying firm Cap Square Solutions employs former Mandel aide Joe Ritter. Ritter declined to comment to ProPublica about his role with Norris’ lobbying firm or whether he is involved with the Government Integrity Fund. The race between Brown and Mandel is considered vital to Republicans who want to take control of the Senate and Democrats who want to hold on to their majority. It has turned into Ohio’s — and the nation’s — most expensive race. The Associated Press reported in August that outside groups — like the Government Integrity Fund — have spent $15 million supporting Mandel, while similar groups have spent $3 million for Brown. It’s unknown where the money is coming from because federal regulations and the U.S. Supreme Court’s Citizens United case allow the groups to spend unlimited amounts of cash on political ads without disclosing their donors. Such groups are classified as non-profit “social welfare” groups, which don’t have to release donor information or register with the Federal Election Commission. They’re supposed to be “primarily” engaged in promoting social welfare. Super PACs aren’t supposed to coordinate with campaigns, but it is common for them to hire politicians’ former aides. According to ProPublica, Ritter was first hired by Mandel as an aide when the candidate was in the Ohio Legislature. He was then the field director for Mandel’s state treasurer campaign and then became a constituent and executive agency liaison when Mandel won that race. He left the treasurer’s office after six months to work for Norris’ lobbying firm. Ritter was part of an ethics complaint filed after a Dayton Daily News investigation into Mandel’s practice of hiring former campaign workers for state jobs. Ritter has contested the charges.Norris' ties to the Government Integrity Fund was discovered by ProPublica through documents filed with Cincinnati NBC affiliate WLWT. The Federal Communication Commission requires TV stations to keep detailed records about political advertisers.