SIOUX FALLS, S.D. — South Dakota's top federal prosecutor is joining his North Dakota counterpart in leaving those posts and opening local offices for a large law firm known for winning big settlements in high-profile litigation.

Brendan Johnson, who has been U.S. attorney for South Dakota since 2009, announced his resignation Wednesday, a week after North Dakota's U.S. attorney, Tim Purdon, said he would be stepping down.

Department of Justice rules prevent Johnson or Purdon from saying yet where they are going, but a person with knowledge of the moves who was not authorized to discuss the situation publicly confirmed to The Associated Press that both will join Minneapolis-based Robins Kaplan LLP and open offices in Sioux Falls and Bismarck. Johnson, 39, leaves his job March 11 and Purdon, 46, on March 12.

"It's a job I've loved and has been a really important part of my life," Johnson said. "I think the work we've done has made a difference in people's lives and I'm really proud of that."

Robins Kaplan, which represents some of the biggest companies in the Upper Midwest, has more than 220 attorneys with offices in Minneapolis and five other cities. The firm won a $7.1 billion settlement in 1998 on behalf of the state of Minnesota and Blue Cross Blue Shield of Minnesota in a lawsuit against the tobacco industry. It also helped forge a $7.25 billion antitrust settlement in 2012 for merchants that accept Visa and MasterCard from the two payment card networks and major banks that issue the cards.

The firm did not return a call seeking comment.

Purdon said Wednesday that he and Johnson used the same legal recruiting firm and their head hunter was Jane Roberts, wife of U.S. Supreme Court Chief Justice John Roberts. Her representative did not immediately return a call seeing comment.

"Working closely with Brendan over the past five years on Indian Country public safety issues has been the highlight of my professional life," Purdon told the AP. "As we discussed our futures, the possibility of continuing our partnership in the private sector was something we wanted to do and we worked hard to make it happen."

Some Democrats had hoped Johnson would run for the Senate seat left open when his father, U.S. Sen. Tim Johnson, retired in January. Johnson declined to say on Wednesday whether he eventually would run for public office.

"During my time as U.S. attorney, I haven't talked about politics and I haven't answered any questions about my political future," Johnson said. "Certainly, those questions will be asked by some after I'm done as U.S. attorney, but as long as I am the U.S. attorney, I do not want to engage in that kind of speculation."

President Barack Obama nominated Brendan Johnson to be South Dakota's U.S. attorney and the Senate unanimously confirmed him in October 2009. Prior to that, Johnson was a federal law clerk in Rapid City for U.S. District Court Judge Karen Schreier, a Minnehaha County deputy state's attorney in Sioux Falls and a partner in the Sioux Falls law firm now known as Johnson, Abdallah, Bollweg and Parsons LLP.

He graduated from the University of South Dakota and the University of Virginia School of Law.

Johnson said that when he took office the state had never prosecuted a commercial sex trafficking case at the federal level and now it has sent more people to prison for that crime than any other federal district in the country. The office also has had more people receive mandatory minimum sentences for trying to buy sex with a child over the Internet than any other district.

He said he's also proud of his work on Indian Country and the sharp increase in prosecutions, especially on the Pine Ridge and Rosebud reservations. He was chairman of the Native American Issues Subcommittee from 2009-2013 at the request of Attorney General Eric Holder, who praised Johnson for his leadership, advocacy for people and public service.

"As a lawyer and as a leader, Brendan has set a standard of excellence that will not soon be surpassed," Holder said in a statement. "Particularly with regard to public safety challenges on tribal lands, he has served as a key adviser to senior Justice Department officials — including me."

First Assistant United States Attorney Randy Seiler will take over the office until the Senate appoints a new U.S. attorney. In North Dakota, assistant federal prosecutor Chris Myers will take over as acting U.S. attorney.