Britons and the rest of the world first learned of many details from the investigation of Monday's terrorist attack in Manchester from American news outlets. This caused an unusual clash between Britain and the United States.

British Prime Minister Theresa May said Thursday morning that she planned to make clear to President Trump that “intelligence that is shared between our law enforcement agencies must remain secure.” Trump later issued a statement calling the leaks “deeply troubling” and promised an investigation.

One official told The Washington Post that the leaks were considered so serious that British police investigating the Manchester attack would withhold information from the United States. But by late Thursday evening, police said they had resumed intelligence sharing after being given “fresh assurances.”

To U.S. readers, the furor over the leaks may seem surprising. But the situation highlights differing ideas about the relationship between intelligence agencies and media outlets.

It's certainly clear that a number of important details of the investigation first appeared in U.S. media outlets. CBS News and the Associated Press were the first to identify the bombing suspect, Salman Abedi, citing U.S. officials. Later, the New York Times published forensic photographs from the crime scene that gave clues into the explosive device used. It also published a graphic showing where victims' bodies were found.

Secretary of State Rex Tillerson condemned the release of information about the Manchester bomb attack investigation on May 26. (Reuters)

The Times story, in particular, caused anger in some quarters of the British media world. Channel 4's Jon Snow wrote on Twitter that it was the “price of sharing intelligence with Trump's America.” However, the idea that the U.S. intelligence community was more likely to leak information to journalists than their British peers is hardly new.

“As a matter of precedent, you see far fewer leaks coming out of the intelligence services in this country on issues of terrorism and national security than you do in U.S.,” said Shashank Joshi, a senior research fellow at the Royal United Services Institute in London.

There have been some similar cases in the past. After London transit bombings in 2005, important details about the blasts appeared in U.S. media after a briefing given by the New York Police Department to security experts. At the time, New York Police Commissioner Raymond Kelly said he had mistakenly believed that London's Metropolitan Police had cleared the details for publication.

The NYPD case highlights one of the key differences between Britain and the United States on the issue of intelligence: The U.S. intelligence community, consisting of myriad agencies, is simply far larger and more complex than Britain's, which means there is more space for both deliberate and inadvertent leaks.

Another difference is perhaps more philosophical than practical: Whereas the First Amendment officially protects the American press, British journalists have more tenuous legal protection.

For decades, Britain's intelligence agencies have issued official requests asking media outlets to not publish certain details for reasons of national security. Such a request is known officially as a DSMA-Notice but is more commonly referred to as a D-Notice.

Calder Walton, a British lawyer studying at Harvard's Kennedy School of Government, says that in the past D-Notices had been an “astonishingly successful” system, despite no real legal standing. “It's not a legal requirement, but it's custom or obligation that's respected within Fleet Street,” Walton said, referring to the London street synonymous with Britain's news industry.

Part of the reason for this system, Walton said, was the historical secrecy surrounding Britain's intelligence agencies. It was only in 1989 that the domestic counterintelligence agency MI5 was recognized in law, while it took foreign intelligence agency MI6 five more years to get recognition. For decades, officials from these organizations did not appear in public and had limited contact with the media.

In the aftermath of the attack, it appears that at least some British journalists appeared to know the attacker's name ahead of its publication. According to Geoffrey Dodds, secretary of the Defense and Security Media Advisory Committee, two D-Notices have been issued in relation to the attack in Manchester, though Dodds said that neither concerned the name of the attacker or the images published by the New York Times.

David Omand, former director of Britain's Government Communications Headquarters and a visiting professor at King's College London, said that it is generally standard practice in Britain to ask the media to hold back the name of a terror suspect for 36 hours so that their associates can be traced. “U.K. media routinely cooperates with this request,” Omand wrote in an email. “U.S. media play by different rules.”

The D-Notice system has long had its limitations. It first came to be widely known in 1967, when Prime Minister Harold Wilson accused the Daily Express of breaching two D-Notices. A later inquiry cleared the newspaper of wrongdoing, though the government refused to accept its findings. The system has faced several crises since and has been amended a number of times.

But in an increasingly global media landscape, Britain's system of tight-lipped intelligence agencies and understanding journalists has begun to look ineffectual. In January, after the publication of a dossier alleging links between Trump's electoral campaign and Russia, a D-Notice was issued to protect the identity of the former British spy who had written the document. The name was then published by a U.S. publication: the Wall Street Journal.

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