WASHINGTON – The nomination of Minnesota U.S. Attorney B. Todd Jones to head the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) is running into stiffening GOP resistance in the U.S. Senate.

Republicans on the Senate Judiciary Committee requested Tuesday that top federal law enforcement officials in Washington turn over documents that might reveal "conflicts" with Jones, President Obama's pick to become the permanent head of the ATF, a position Jones holds on an interim basis.

The requests were made in letters from Iowa Sen. Charles Grassley, the ranking Republican on the committee, to the heads of the Federal Bureau of Investigation, the Drug Enforcement Administration and U.S. Immigration and Customs Enforcement (ICE).

Jones had no comment on the request, a spokeswoman for the U.S. attorney's office said. The Judiciary Committee is expected to approve his nomination next week. But no date has been set for a final Senate vote, which could be subject to a Republican filibuster.

The latest letters, obtained by the Star Tribune, allude to three unnamed witnesses, presumably all law enforcement officials in Minnesota, who reportedly talked privately to Grassley's staff. Sources close to the matter say they are afraid to come forward for fear of retaliation. Nevertheless, Grassley cited them to contradict Jones' testimony before the Senate last month, in which Jones said he was unaware of any "deterioration" in relations between his office and state law enforcement agencies.

Grassley and other Republicans on the committee have faulted Jones for not providing adequate support to law enforcement in cases involving drugs, gangs and violent crimes, particularly gun crimes. Jones, in testimony before the committee, said he has tried to focus limited prosecutorial resources on violent criminals and other high-profile cases. His nomination, made in the aftermath of the schoolhouse shooting massacre in Newtown, Conn., also has been caught up in the broader debate over gun control, where the ATF's regulatory role has come into question.

Jones, an ex-Marine, was brought in to clean up the ATF after a botched gun-tracking operation on the Mexican border known as "Fast and Furious." But his management style has been called into question at the ATF and in the U.S. attorney's office in Minneapolis, which was riven with divisive office politics even before he got there.

The Grassley letters refer indirectly to a whistleblower complaint brought against Jones by Jeffrey Paulsen, an assistant U.S. attorney in Minneapolis, who says he was unfairly disciplined by Jones for raising management concerns. Grassley has demanded that Jones' nomination be put on hold while that case is in mediation.

According to Grassley's letters, his staff also talked by telephone last week with Donald Oswald, the former head of the FBI office in Minneapolis, who said that he had "personally" talked to Jones about the complaints against his office.

In a June 11 appearance before the Judiciary Committee, Jones said he had been "surprised" by a letter Oswald had submitted previously to the Senate that said Jones had an "atrocious professional reputation" in Minnesota.

"My perception," Jones said, "was that we had a professional working relationship."

Oswald's judgment of Jones also was disputed by former U.S. Attorney Tom Heffelfinger, who suggested that Oswald, who has since moved to Florida, was not in Minnesota long enough to assess Jones' reputation.

In reply to a previous follow-up question from Grassley, Jones pointed out that his nomination has been backed by public letters of support from numerous leaders in Minnesota law enforcement, including Heffelfinger, appointed by former President George W. Bush, and Minnesota Bureau of Criminal Apprehension agent Michael Campion, who served as public safety commissioner under former Republican Gov. Tim Pawlenty.

Minnesota Democrats Amy Klobuchar and Al Franken, both members of the Senate Judiciary panel, have supported Jones. White House officials said Tuesday they stand behind Jones, reprising comments made by press secretary Jay Carney in a June 10 briefing. "They should stop coming up with excuses to obstruct a highly qualified nominee and finally put a confirmed director at the helm of a critical law enforcement agency that helps protect our communities from violent criminals, gun crime, and acts of terror." Grassley's three nearly identical letters are addressed to FBI Director Robert Mueller, ICE Director John Morton and DEA Administrator Michele Leonhart, a St. Paul native who once worked out of the DEA's Twin Cities office. All three are being asked to turn over any communications they have suggesting "conflicts" and "expressions of dissatisfaction" with Jones and others in the U.S. attorney's office in Minnesota over what cases to prosecute.

Grassley also has requested the FBI make available J. Chris Warrener, the head of the Minneapolis FBI office, and former acting head Richard Thornton.

Follow Kevin Diaz on Twitter at @StribDiaz.