B. Todd Jones, who won a bruising U.S. Senate fight two years ago to become the first permanent director of Alcohol, Tobacco, Firearms and Explosives (ATF), announced on Friday that he was resigning, effective March 31.

The ATF said the former U.S. attorney for Minnesota planned to "pursue opportunities in the private sector."

Sources told the Star Tribune he is expected to go to work for the National Football League, but it is unclear in what capacity.

"I will truly miss leading and working side-by-side with these men and women in their pursuit of ATF's unique law enforcement and regulatory mission," Jones said in a statement on the ATF website.

Asked about working for the NFL, Jones, 57, said in a text message to the Star Tribune, "I cannot confirm anything until I am gone on 3/31."

Attorney General Eric Holder said Friday that Jones "has cemented his reputation as an exemplary leader, a consummate professional, and an outstanding public servant."

Some associates of Jones were caught off guard by the resignation. But Thomas Kayser, a friend and former partner at Robins, Kaplan, Miller and Ciresi law firm, where Jones once worked [it's now Robins Kaplan] said Jones had been contemplating a change. "I knew he was looking around because he had been in public service" for years "and he wanted to try something else."

Another friend said Jones had been considering a departure from the ATF since last fall, similar to other members of the Obama administration who are leaving as the president's second term winds down.

Jones was U.S. attorney in Minneapolis from 1998 to 2001, and again in 2009. He was named acting ATF director in 2011 and nominated for the permanent post, remaining U.S. attorney until he was confirmed.

The ATF nomination got bogged down in the Senate Judiciary Committee where Sen. Charles Grassley of Iowa, the ranking Republican, tried to block it.

Jones faced questions about a whistleblower complaint filed by a member of his U.S. attorney's staff who claimed he was mistreated, and criticism from a former special agent in charge of the Minneapolis FBI office.

As the confirmation process stalled, Jones considered withdrawing, he revealed much later, but hearkening back to his past as a Marine, he soldiered on. "Marines do not quit," he said.

Sen. Amy Klobuchar, a member of the Judiciary Committee, played a key role in negotiating an agreement that got Jones' nomination approved, then shepherded the nomination through the Senate, making him the first ATF director in history to receive Senate confirmation.

Former U.S. Attorney Tom Heffelfinger expressed surprise Jones was leaving. "I think Todd Jones has done a great job," he said. "He has stabilized the agency and put it on a good direction. I am kind of disappointed that he is leaving. I think Todd gave it stability."

U.S. Sen. Al Franken issued a statement, saying, "I am sad to see him leave. Mr. Jones has been a great public servant, and I wish him all the best in his future endeavors."

Klobuchar praised Jones' work as ATF director and U.S. attorney in a separate statement, noting she worked with him when she was Hennepin County attorney.

"While Todd is leaving his position before the end of the president's term, the fact that the Senate was able to come together to confirm an ATF director for the first time in seven years will help pave the way for future confirmations," she said.

"Before he was nominated and we worked so hard to get him confirmed, people actually thought that no one could ever be confirmed for the job."

Holder cited "bold changes" Jones had made at the ATF "including ballistic imaging technology." He also said that under Jones' guidance, the ATF implemented its "Frontline business model — a data-driven approach designed to ensure the agency can focus its resources to achieve maximum impact."

Jones took over the ATF after the "Fast and Furious" scandal in which the agency failed to properly track weapons purchased along the nation's Southwest border. The weapons sometimes ended up in the hands of criminals and in one case an untracked gun was used to kill a U.S. Border Patrol agent.

Earlier this month, the ATF was again under political fire.

The gun rights lobby objected to an end-to-a-rule exemption that allowed the sale and manufacture of certain types of bullets used in the popular AR-15-style rifles that could pierce a police officer's protective vest when fired from a handgun, the Associated Press reported.

Most of the 80,000 public comments about the proposal were critical of the plan, and dozens of U.S. senators and 237 representatives signed a letter calling the effects of the ATF proposals "untenable. "

The proposal was withdrawn by the ATF.

However, a friend said Jones was planning to leave the agency for months, and the dispute over ammunition rules had nothing to do with it.

ATF Deputy Director Thomas Brandon will serve as acting director after Jones departs, the ATF said Friday. Brandon was appointed ATF deputy director in October 2011.

Staff writer Jim Spencer contributed to this article.

Randy Furst • 612-673-4224

Twitter: @randyfurst