WASHINGTON — Secretary of State Mike Pompeo on Thursday released a letter written four months ago from an independent federal investigative agency that said he did not violate the law when he explored a Senate run from Kansas, escalating a battle with a senator who accused him of using State Department resources to further his political ambitions.
The letter was written by the United States Office of Special Counsel, which takes up whistle-blower complaints and investigates charges that officials have violated the Hatch Act, a 1939 law limiting political activities of federal employees.
“Based on our review of the evidence to date,” the letter said, the office “cannot conclude that you are currently a candidate in the 2020 Senate election in Kansas.” It told Mr. Pompeo that the office was “closing this matter but reserves the right to reopen its investigation pending any new developments.”
In making its determination, the investigators relied on Mr. Pompeo’s assertions in early January that he was not running for an open Senate seat in his adopted home state.
In the fall, Senator Robert Menendez of New Jersey, the top Democrat on the Senate Foreign Relations Committee, asked for a special counsel investigation into whether Mr. Pompeo was violating the Hatch Act with his frequent trips to Kansas in 2019, which critics say amounted to a shadow campaign for the Senate seat. Three of the four visits were official taxpayer-funded trips, and during one in October, Mr. Pompeo quietly met with Charles G. Koch, the Republican billionaire donor.
The letter was dated Jan. 21, but Mr. Pompeo made no reference to it last week, when he curtly dismissed charges that he had used taxpayer funds to have a federal employee run personal errands for him and his wife, Susan Pompeo.
Those charges were at the center of an investigation opened by the inspector general of the State Department, Steve A. Linick, whom President Trump fired this month. Mr. Pompeo has refused to say why he recommended Mr. Linick’s dismissal.
Mr. Pompeo sarcastically dismissed the investigations last week. “I’ve seen the various stories that someone was walking my dog to sell arms to my dry cleaner,” he said. “I mean, it’s all just crazy. It’s all crazy stuff.”
Usually letters like the one Mr. Pompeo released are not made public, and he did not say why he had waited to unveil it. But he sent a copy of the letter to Mr. Menendez on Thursday, and included a number of reporters who have covered the issue.
“You have shown an oversized interest in a handful of trips that I took to Kansas,” he told Mr. Menendez, with whom he has had an escalating feud. “The O.S.C. response to your hackery makes clear your continued effort to politicize legitimate and important diplomacy and national security activity was without merit.”
He accused Mr. Menendez of “generating a continuing series of media articles and reports with your rumors, innuendo and flat untruths about me,” and noted that he had “copied the reporters who joined in your slander on this.”
Mr. Menendez issued a statement in response, saying, “Clearly the secretary of state feels deeply disturbed by the ongoing oversight work of the Senate Foreign Relations Committee.”
“High level temper tantrums will not stop the committee from conducting our oversight responsibilities,” he added.
Mr. Pompeo’s release of the letter occurred right before a June 1 deadline to declare intent to enter the Senate race in Kansas. Since the winter, Mr. Pompeo had tried to dispel notions that he would run for the seat, but speculation continued this week, in part because Republican leaders kept asking Mr. Pompeo to do so, given that control of the Senate is at stake in November.
Mr. Trump had also kept the speculation alive, telling Fox News late last year that “if he thought that there was a chance of losing that seat, I think he would do that and he would win in a landslide because they love him in Kansas.”
After Mr. Linick’s dismissal, news organizations reported on a series of lavish dinners that Mr. Pompeo and his wife had hosted at the State Department, where guests included prominent Republican donors, lawmakers and personalities in the news media. But there is no indication yet that those so-called Madison Dinners prompted an investigation by Mr. Linick, and Mr. Pompeo’s defenders argue they did not differ much from the practices of his predecessors, including Hillary Clinton.
Mr. Menendez’s office had heard complaints from within the State Department about the dinners and had been looking into them for months. On May 19, the senator sent a private letter to Stephen E. Biegun, the deputy secretary of state, asking for answers to questions about Mr. Pompeo and the dinners. The next day, Mr. Pompeo lashed out at Mr. Menendez during a news conference, though he did not mention Mr. Menendez’s query about the dinners.
Days earlier, on May 16, Mr. Menendez and Representative Eliot L. Engel, Democrat of New York and the chairman of the House Foreign Affairs Committee, announced they were opening an inquiry into the firing of Mr. Linick.
Part of the scrutiny of Mr. Pompeo involves whether he is using State Department funds for activities that help him build up a political network for a potential presidential campaign in 2024. Mr. Pompeo has quietly visited with Republican donors and political figures on diplomatic trips, leaving those off his public schedule.
The second investigation by Mr. Linick examined whether Mr. Pompeo and other top administration officials acted illegally in declaring an “emergency” last year to bypass a congressional freeze on arms sales to Saudi Arabia and the United Arab Emirates, which have carried out a devastating air war in Yemen. That investigation was close to completion.
Mr. Pompeo had declined to be interviewed by Mr. Linick’s investigators on the arms sales matter but had submitted written answers to questions, indicating he knew of the inquiry and its focus. And in early March, Mr. Linick’s employees briefed senior department officials on the preliminary findings.
Lara Jakes contributed reporting.