Attorney General William P. Barr said Monday that he did not expect the prosecutor he handpicked to review the 2016 FBI investigation into President Trump’s campaign would investigate former president Barack Obama or former vice president Joe Biden — an assertion likely to dismay Trump and his conservative allies.

Barr’s comment came at a news conference to discuss last year’s shooting at a U.S. military base in Pensacola, Fla. A reporter asked about Trump suggesting publicly in recent weeks that top officials in the Obama administration, including the former president, had committed crimes.

While noting he was not taking aim at Trump’s comments specifically, Barr lambasted what he called the “increasing attempts to use the criminal justice system as a political weapon.”

“The legal tactic has been to gin up allegations of criminality by one’s political opponents based on the flimsiest of legal theories,” Barr said. “This is not a good development.”

He then specifically dismissed the idea that Connecticut U.S. Attorney John Durham — who has been examining how the FBI handled the 2016 investigation into possible coordination between the Trump campaign and Russia to influence the election — would look at Obama or Biden.

“As to President Obama and Vice President Biden, whatever their level of involvement, based on the information I have today, I don’t expect Mr. Durham’s work will lead to a criminal investigation of either man,” Barr said. “Our concern over potential criminality is focused on others.”

He declined to say who Durham was focused on.

Trump and his allies have begun suggesting Obama and other top officials in his administration broke the law during the Russia investigation — a conspiracy theory they have dubbed “Obamagate” — particularly in their treatment of former Trump national security adviser Michael Flynn. Barr’s comments seem to suggest, though, that he does not want the U.S. law enforcement apparatus to be drawn in to another political squabble.

“We cannot allow this process to be hijacked by efforts to drum up criminal investigations of either candidate,” Barr said, referring to Trump and Biden, Trump’s presumptive Democratic opponent.

Barr has in the past sought to publicly warn Trump about getting too involved in Justice Department business. In February, for example, he went on ABC News to declare Trump’s tweets about Justice Department business “make it impossible for me to do my job,” and the attorney general privately told people close to Trump he had considered resigning over the matter. Barr also issued formal guidance to the Justice Department earlier this year that any investigations of presidential candidates first need his approval.

But Barr’s words have sometimes outstripped his actions. His ABC News interview came after Trump had tweeted about the criminal case against his longtime friend Roger Stone, just before Barr personally intervened in the case to reduce the sentencing recommendation that career prosecutors had given to a judge. Despite Barr’s warning, Trump has continued to tweet and talk about Justice Department cases, and Barr has remained attorney general.

The attorney general also has given several U.S. attorneys special commissions to look into matters of interest to Trump.

Durham, for example, is broadly exploring the Russia investigation. Jeff Jensen, the U.S. attorney in St. Louis, is examining the prosecution of Flynn. Scott W. Brady, the U.S. attorney in Pittsburgh, was tasked with taking information from Trump’s personal lawyer, Rudolph W. Giuliani, and others wanting to provide the Justice Department with material alleging wrongdoing related to Biden, his family and their dealings in Ukraine.

Trump has long wanted criminal charges against those he considers political rivals — especially former FBI director James B. Comey and former acting FBI director Andrew McCabe — and has been frustrated that Barr’s Justice Department has been unwilling to bring such cases.

In recent weeks, Trump has seized on developments in the Flynn case to turn his attention to Obama and Biden. At Jensen’s recommendation, the Justice Department moved to undo Flynn’s guilty plea to lying to the FBI about his conversations with a Russian diplomat.

U.S. District Judge Emmet Sullivan is considering that matter and has appointed a retired federal judge to oppose the Justice Department’s position. Trump told Fox News that he had “read legal scholars” and they “can’t believe” what Sullivan was doing to Flynn.

On the same day of the Justice Department’s move to drop Flynn’s case, Trump’s top intelligence adviser, Richard Grenell, visited the Justice Department to deliver a list — which he had just declassified — of former Obama administration officials including Biden, who made requests during the presidential transition that would ultimately “unmask” Flynn’s name in National Security Agency files.

There is nothing wrong with such requests: Unmasking is a routine practice used to identify U.S. individuals who are referred to anonymously in intelligence documents, and it is meant to help government officials better understand what they are reading. But Trump and his supporters seized on the documents to allege wrongdoing.

Trump last week urged Senate Judiciary Committee Chairman Lindsey O. Graham (R-S.C.) to call Obama to testify about what he dubbed “the biggest political crime and scandal in the history of the USA.” Graham cast doubt on whether that would be a good idea. On Monday, he announced his committee would vote on whether to subpoena testimony or materials from several law enforcement officials involved in exploring Russian interference, including Comey, McCabe and former CIA Director John Brennan.

Trump has been unable to articulate what crime he thinks has been committed.

“You know what the crime is,” he told a reporter who pressed him for details last week.

More recently, Trump suggested to Fox News, without evidence, that “treason” had been committed, and more forcefully reiterated his calls to prosecute those he considers political foes.

“People should be going to jail for this stuff,” he said, adding later, “This was all Obama. This was all Biden. These people were corrupt. The whole thing was corrupt. And we caught them. We caught them.”

Even as Barr said Durham would probably not investigate Obama and Biden, he seemed to add fuel to some of Trump’s long-standing attacks on the Russia investigation, asserting that law enforcement and intelligence officials were involved in “a false and utterly baseless Russian collusion narrative against the president.”

“It was a grave injustice, and it was unprecedented in American history,” Barr said.

But, referring to the recent Supreme Court decision to overturn the convictions of two of former New Jersey governor Chris Christie’s political allies in the “Bridgegate” case, Barr asserted, “there’s a difference between an abuse of power and a federal crime.”

“It cannot be and it will not be a tit-for-tat exercise,” Barr said.