With British officials incensed that intelligence leaks to U.S. media aired details about their investigation of last week’s deadly Manchester bombing, President Trump has promised to find those responsible for disclosing that and other sensitive information.

But getting to the bottom of who is behind the leaks may be easier said than done.

Leak investigations generally start by process of elimination, and the more people who had access to the leaked information, the trickier it is to pinpoint the source, say former FBI officials and government secrecy experts.

“Investigations, usually from the FBI, will determine who in the first place had access to the leaked information,” said Steven Aftergood, a government secrecy specialist with the Federation of American Scientists. “If it’s a very large number, that might be the end of the matter because it might simply be too hard to narrow it down through ordinary investigative techniques.”

If only a reasonable number of people are determined to have had access to the leaked information, investigators may go ahead with interviews, polygraph tests, or otherwise try to correlate potential contact between suspects and the reporters or news outlets who published the leaks, Mr. Aftergood said.

That was the route investigators initially took when they tried to identify the source of a 2012 leak to The Associated Press about a CIA operation that disrupted a plot by al Qaeda in the Arabian Peninsula to detonate a bomb on an airplane. The FBI had interviewed more than 550 officials about the leak but had not been able to identify the source until agents subpoenaed call records for 20 phone lines assigned to Associated Press bureaus and reporters.

The investigation set off a firestorm of controversy for news organizations, with the Obama administration criticized for bringing more leak cases — at least eight prosecutions — than all predecessors combined.

The scrutiny prompted then-Attorney General Eric Holder to revise Justice Department guidance that tightened protocol for cases in which federal investigators can subpoena and seize reporters’ communications records, requiring authorization by the attorney general or other designated senior DOJ officials. The revision came in 2015 as the Justice Department abandoned efforts to force a New York Times reporter to testify in the trial of a former CIA officer accused of disclosing classified information.

But the policy is just that — departmental guidance that can be changed from one administration to the next.

Given Mr. Trump’s and Attorney General Jeff Sessions’ insistence on containing the leaks, some wonder whether that policy may change.

The New York Times reported earlier this month that one of the conversations Mr. Trump had with FBI Director James B. Comey before firing him included a discussion about leaks to the news media and the president’s suggestion that the FBI consider jailing reporters for publishing classified information.

“Based on some of the public statements both the attorney general and Mr. Trump have made, it would appear likely this DOJ would have a different approach to prosecution of the media than the prior attorney general’s office,” said Mary-Rose Papandrea, a law professor at the University of North Carolina and expert in national security leaks. “The media community has been afraid of what kind of guidance this DOJ will release regarding its treatment of the press and what kind of policy it will have in place for subpoenas for reporters or to third parties.”

Mr. Sessions announced last week that such intelligence leaks would not be tolerated and that the DOJ had “already initiated appropriate steps to address these rampant leaks that undermine our national security.”

A DOJ spokesman declined to elaborate on what steps have been taken.

The British blame U.S. officials for the leak of the Manchester suicide bombing suspect’s name to American media, as well as photos of the detonated device. They briefly stopped sharing information with U.S. agencies as a result.

In terrorism cases, intelligence is shared with partner agencies to keep them apprised of potential threats and to produce leads. The FBI often briefs law enforcement partners that are part of its Joint Terrorism Task Force on an incident, said Ron Hosko, a former assistant FBI director. If U.S. officials suspect the leak sprung from such a partnership, they may reassess their information-sharing practices, he said.

“It is destructive to the trust between the Brits and U.S. but also internally, to the Joint Terrorism Task Forces, to have the FBI looking suspiciously around the table,” Mr. Hosko said.

But that isn’t to say all leaks are bad — such is the case with whistleblower claims.

“There are unauthorized disclosures of classified information that serves the public interest by revealing misconduct, illegal activity, or stupidity or other kinds of government action that people in a democracy are entitled to know,” Mr. Aftergood said. “We don’t want to get to a situation where leaks are literally impossible because sometimes they are desirable.”

The Manchester case is hardly the only ongoing leak investigation. The Trump administration has been inundated with leaks throughout the four months the president has been in office — with details of Mr. Trump’s phone conversations with his counterparts in Australia and Mexico disclosed as well as his Oval Office meeting with Russian Foreign Minister Sergey Lavrov.

The House Permanent Select Committee on Intelligence has also opened leak investigations, though its unclear what progress they’ve made. Meanwhile, officials have identified three leakers of classified information at the White House who are expected to be fired, CBS News reported.

But even if investigators from any of the probes track down the source of a leak, other issues may stop them short of criminally charging a suspect.

A probe might not yield the evidence required to bring a case to court, though it may result in enough to take administrative action, such as issuing a reprimand or firing the person, Mr. Aftergood said.

Counterintelligence professionals have also complained that the stature of a leaker may also play a role.

“They may have down to a certainty who the leaker was, but if the leaker was a senator or congressman there is this reticence by DOJ to bring this,” Mr. Hosko said.

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