LONDON -- Throughout the long months of the Leveson inquiry into the behavior and standards of the British press, the riddle that permeated its scrutiny of the nation's newspapers was this: If journalists cast themselves as protectors of public probity, who guards the guardians?
A comparable question now resonates for the authorities themselves: If Britain's security apparatus is designed to shield the land against threat, who will ensure that the laws that empower them to defend against terrorism are not abused?
The concerns surfaced after the nine-hour detention, while transiting Heathrow Airport, of David Miranda, the partner of the journalist Glenn Greenwald who has reported for The Guardian on disclosures by the former National Security Agency contractor Edward J. Snowden into the agency's intelligence-gathering.
Mr. Miranda was held under contentious laws supposed to protect Britons from terrorism, provoking worries that the legislation -- Schedule 7 of the Terrorism Act 2000, to give it its full name -- was being deployed without justification.
The alarms sounded yet louder when Alan Rusbridger, the editor of The Guardian, reported that Britain's equivalent of the N.S.A., the Government Communications Headquarters, or GCHQ, had sent two operatives to his offices to oversee the destruction of computer hard drives.
The notion -- government spies in a newspaper office supervising cyber-censorship -- seemed inconceivable to some Britons, their actions likened by the Guardian columnist Simon Jenkins to "book burners sent by the Spanish Inquisition."
When press freedoms are threatened, journalists are the first and the loudest to cry foul. But the events surrounding Mr. Miranda's detention raised far broader issues about the nature and motives of those who cite national security as the imperative of secrecy.
"Suspicion is growing," the left-leaning Daily Mirror said in an editorial, "that the Establishment, including 'securocrats' who want to be free to bug and tap without scrutiny, are intimidating journalists in the hope of evading public accountability."
Official and hidden controls on the press are by no means new in Britain. A system of unpublished edicts -- known as D-Notices -- has gagged reporters for decades. As the Leveson inquiry confirmed, cozy ties between the police, the press and politicians swayed news coverage.
But the latest disclosures coincide with a deepening mistrust of authority, particularly the police, who are already facing questions about the integrity of undercover activities in political and criminal investigations. "We live in an age in which government is held in contempt and transparency venerated," the columnist Matthew d'Ancona wrote in The Evening Standard.
The disclosures by Mr. Snowden about cooperation between GCHQ and the N.S.A. have deepened suspicions, moreover, that the "special relationship" with the United States of which successive British leaders have been so proud is, in fact, little more than a slavish kowtowing to America, forfeiting older, libertarian values.
"In Britain we live in the oldest unbroken democracy on earth -- not a police state," said Shami Chakrabarti, the head of the civil rights group Liberty. "And yet the increasing power given to police under the Terrorism Act has passed under the radar."
In the process, Britain's quest for moral high ground from which to judge others has been undermined. If Mr. Miranda had been seized "by security services in Moscow or Tehran, ministers would be accusing foreign powers of taking liberties," The Daily Mirror said. Instead, "this happened in London, so it is our government with questions to answer."
As Mr. Jenkins wrote in The Guardian, "two great forces are now in fierce but unresolved contention."
The "modern state," he wrote, "is gathering, storing and processing for its own ends electronic communication from around the world," while "this power has so corrupted those wielding it as to put them beyond effective democratic control."
There is a counterargument illuminating not "the scary power of 21st century government but its scary weakness," wrote Mr. d'Ancona. "Digital technology has made it alarmingly easy to seize, to store and to disclose at whim extraordinarily sensitive information about the activities of those tasked with protecting our collective security."
That was as true of Mr. Snowden and Mr. Greenwald as it was of Bradley Manning and Julian Assange.
British operatives may have acted in ways described variously as counter-productive, crude, clunky and intimidatory, in other words, but their actions illuminate the increasingly nuanced challenges they confront in seeking to square a circle of secrecy, civil rights and national security.
In recent years, the authorities have sought to balance their apprehensions about the intentions and motives of individual suspects -- often drawn from clandestine surveillance -- against the call for due process in dealing with them.
An array of conspiracies have been thwarted, and plotters have been prosecuted in court. And, at every turn, the government has invoked its right and responsibility to protect the country from terrorism -- an argument echoed by the home secretary, Theresa May, in Mr. Miranda's case.
But the authorities have not established a link between his nine hours in detention -- and the seizure of his electronic equipment -- and a defined terrorist threat.
"Arbitrary powers bring huge dangers of injustice," said Ms. Chakrabarti, referring to the counterterrorism laws. "When these things are passed, people think it will never happen to them." But episodes like the Miranda affair "make us realize anybody could be targeted."
This article originally appeared in The New York Times.