Ten years ago the late Martin McGuinness, then deputy first minister at the Northern Ireland assembly, stood beside his unionist counterpart Peter Robinson, the Democratic Unionist party first minister, and called the dissident republicans who had murdered a police officer “traitors to the island of Ireland”. Two weeks ago a senior Sinn Féin figure on the Northern Ireland Policing Board said that among the Catholic community (nationalist and republican) confidence in the Police Service of Northern Ireland (PSNI) had hit rock bottom. Now Karen Bradley, the secretary of state, has trampled into the middle of this crisis and made matters a great deal worse.

Bradley, who said last week that killings during the Troubles by members of the police and the British army were “not crimes”, has the authority to select the person tasked with investigating police wrongdoing, the police ombudsman, when the current incumbent leaves in the summer. This is absurd and untenable. Bradley offered a belated apology, claiming, bizarrely, that she did not mean what she said, and that it was not her view. However, at a Northern Ireland affairs committee hearing several months ago she supported a call to change the law so that army veterans and former police officers “do not face harassment in the courts”.

Things fall apart, and the vision of reconciliation and the sharing of power that was at the heart of the 1998 Good Friday agreement is disintegrating by the day. The wrecking ball that is Brexit still does its work, fuelled by nostalgia. For some this is for the days when the Northern Ireland government was safely dominated by unionism. When the police was a Protestant force with “special powers”. When there was no talk of human rights. For others it is for a time when republicanism meant armed struggle, with the murder and mayhem that entailed, until Ireland was united.

Northern Ireland’s power-sharing executive, created by the Good Friday agreement, collapsed in acrimony two years ago. Late last year a survey found that 87% of those in Northern Ireland who voted for Brexit believed the collapse of the peace process would be an acceptable price to pay. Most of these would be DUP voters. Theresa May’s government is relying on that party’s support and has shown scant regard for the 56% of people in Northern Ireland who voted to remain in the EU.

Dissident republicans rejected the Good Friday agreement. Their idea of policing includes shooting teenage boys in the legs in alleyways and daubing the names of alleged paedophiles on gable walls. They threaten and attack police officers. Sinn Féin’s loss of confidence in the police allows them to claim vindication for the view that nothing has changed, that the police is a sectarian force. The various dissident groupings have little capacity and low support but when all else is failing, they have opportunities to exploit. Riots in the north’s second city, Derry, last year, and the attempted bombing of the city’s courthouse this year, show they can cause significant disruption. They can also appeal to young people who are angry. Angry because they live in areas that were the poorest places before and during the conflict, and remain so. Angry that there are no decent jobs for them and that they are harassed over their entitlements to inadequate benefits. Angry at being subjected to heavy-handed stop and search operations and house searches by the PSNI.

The force is due to appear in Belfast’s high court later this month to make its case for refusing to disclose documents relating to its response to an award-winning documentary, No Stone Unturned, about the 1994 murders of six Catholic men as they watched a World Cup football match at a bar in Loughinisland, Co Down. The film made use of a report by the police ombudsman that found that police had colluded with the loyalist gang responsible, and the investigation that followed had been characterised by “catastrophic failings”. No one has been brought to justice for the murders.

The police didn’t arrest and question the alleged killers, who were named in the film, but did arrest two journalists behind the documentary. Trevor Birney and Barry McCaffrey are now on extended police bail, facing criminal charges of theft and of breaching the Official Secrets Act.

Their lawyers accused the PSNI of trying to “keep a lid” on the case. The PSNI said it arrested the journalists after receiving a complaint of theft of documents by the ombudsman, Michael Maguire. He denies making such a complaint. The case of the journalists is seen internationally as an important test of press freedom. Lawyers for the journalists also argue that the Official Secrets Act is incompatible with article 10 of the European convention on human rights, which gives foundation to the legal rights of journalists.

Some within the PSNI, and many former police officers, resent the activities of the ombudsman. They are furious at what they see as a witch-hunt against a force that was struggling to maintain law and order in the face of ruthless terrorists. They have tried unsuccessfully to have his powers limited. Last month Maguire revealed he had discovered that police had not supplied his investigators with material relevant to his investigation into a series of loyalist atrocities dating back to the 1980s and 90s. This was, he said, “sensitive material, intelligence-led material [including] information on covert policing”.

The PSNI apologised, blaming “human error”, but a solicitor representing some of the victims said the families, most of whom are from the Catholic community, were “appalled”. Some of the reports Maguire had hoped to deliver before his term ends will now be delayed for an unknown period.

The transformation of the police into an impartial force that was representative of and trusted by the whole community was regarded as a mainstay of the post-Good Friday agreement landscape. The old police force, the Royal Ulster Constabulary, was overwhelmingly drawn from the Protestant, unionist community. In 2001 Chris Patten introduced affirmative actions, including a 50/50 quota for new recruits. With these arrangements in place, about 45% of the force should now be drawn from the Catholic community. But the most recent figures show that the ratio is still about 30%. Worse, the vast majority of senior officers are still Protestants. Only eight out of the top 68 posts are held by Catholics. The situation is not improving – only 21% of those training as police officers are Catholic. Those from other backgrounds in Northern Ireland also continue to be underrepresented.

Quotas were dropped in 2011, but the Social Democratic and Labour party has called for them to be restored. The DUP rejected this, claiming they represented “institutionalised sectarian discrimination”. The chief constable, George Hamilton, is also retiring this summer. Sinn Féin’s president, Mary Lou McDonald, caused a crisis when she declared that nobody from the PSNI would be acceptable as his successor. Hamilton retaliated, accusing her of poor leadership and causing damage to both policing and peace. Whatever the balance of blame, neither policing nor peace are in good shape as we lurch towards the possibility of a hard border once again in Ireland.

• Susan McKay is the author of Bear in Mind These Dead, about the legacy of the Troubles