BELFAST, Northern Ireland (AP) — Peter Olphert was 14 when Irish Republican Army gunmen killed his father. Forty years later, he says it’s time to set aside the past.

Mark Thompson lost his brother to British Army bullets, another victim of “the Troubles” that wracked Northern Ireland for three decades. He thinks society can’t move forward until it confronts unfinished business and holds some of those responsible to account.

This month marks 25 years since the Good Friday Agreement largely ended bloodshed that left 3,600 people dead, some 50,000 wounded and thousands bereaved. Northern Ireland is observing the anniversary with a reunion of key peace process players and a visit from U.S. President Joe Biden.

The peace accord may have stopped the fighting, but deep divisions remain over the conflict’s legacy — making it hard for some of Northern Ireland’s 1.9 million people to move past it. And Britain’s exit from the European Union has only complicated matters, creating political tensions that have rattled the foundations of the peace agreement.

“It is time, in my opinion, to draw a line in the sand and move forward,” said Olphert, who recently retired after 30 years as a police officer — the same job held by his father John Olphert, who was shot dead by masked gunmen in 1983 in the family’s shop.

In some ways, Olphert made the decision to move on years ago. He said it would have been “very easy” for him, as a grieving teenager, to join one of the pro-British loyalist militias waging war against Irish republican militants in a neighbor-on-neighbor conflict that also drew in the British military.

“There was that invitation there, let’s just say, that I should follow that certain path and get revenge. But that was never for me,” he said. “The more you perpetuate what happened in the past, the more generations are going to have that bitterness.”

But Thompson argued that for many bereaved families, moving on is not so simple — and moving on without fully grappling with the past could inadvertently set the stage for more conflict.

After his brother Peter was shot dead by undercover British soldiers in Belfast in 1990, he co-founded Relatives for Justice, a group that campaigns to uncover the truth about killings involving U.K. security forces, for which there have been few prosecutions.

“To say that we draw a line under that means that we don’t learn the lessons of it,” Thompson said. “The lesson of any society emerging from conflict is you can’t sweep it under the carpet because … it really reinvigorates some of the grievances that lead to further conflict.”

Ending the Troubles meant balancing competing identities in Northern Ireland, which remained in the United Kingdom when the rest of Ireland won independence a century ago. Irish nationalists in the north — most of them Catholic — seek union with the Republic of Ireland, while largely Protestant unionists want to stay part of the U.K.

The Good Friday Agreement, struck on April 10, 1998, after almost two years of U.S.-backed talks, committed armed groups to stop fighting, ended direct British rule and set up a Northern Ireland legislature and government with power shared between unionist and nationalist parties.

“Today we have just a sense of the prize that is before us,” then-British Prime Minister Tony Blair said on the day the deal was agreed. “The work to win that prize goes on. We cannot, we must not let it slip.”

The peace accord succeeded far better than many expected, despite occasional attacks by dissident armed groups that last month prompted U.K. authorities to raise Northern Ireland’s terrorism threat level to “severe,” meaning an attack is highly likely.

During the Troubles, downtown Belfast was a ghost town at night, surrounded by a security ring of steel. Now busy pubs, hip cafes and microbreweries dot the Victorian streets. A gleaming new campus for Ulster University is helping revive the scarred city center.

Steve Malone, a guide who leads walking tours focused on Belfast’s bloody past, said “people know really only two things when you say Belfast — they think of the Troubles and they think of Titanic,” the doomed ocean liner built in the city’s shipyard.

“It’s a hugely different place now,” he said. “Even in the physical infrastructure. We now have a transport system that connects the western, Catholic-dominated side of the city with the eastern, Protestant-dominated side. That did not happen during the conflict.”

But the threat of violence has never completely disappeared, and Katy Hayward, professor of political sociology at Queen’s University Belfast, said one goal of the peace agreement has been neglected: reconciliation.

She said the deal placed emphasis on releasing prisoners jailed for taking part in the conflict and reintegrating them into society. As a result, former militants “remain powerful and influential” in their communities, often to the exclusion of peacebuilders.

“We never properly dealt with the causes of the situation in which violence is still glorified in some communities,” Hayward said.

A plan by the U.K. government to end prosecutions of both militants and British soldiers for alleged crimes committed during the Troubles would only further bury hopes of holding perpetrators to account. It has been met with widespread opposition.

The possibility of violence is the reason fortified 25-foot- (8-meter-) high “peace walls” still separate some nationalist and unionist neighborhoods in Belfast. Rival murals of masked IRA fighters and gun-toting loyalist militants adorn streets on either side.

Britain’s departure from the European Union, which left Northern Ireland poised uneasily between the rest of Britain and EU member Ireland, has also upset a delicate political balance, including the power-sharing system set up by the peace accord.

The Northern Ireland Assembly has not sat for more than a year, after the main unionist party pulled out of the government to protest new trade rules for Northern Ireland brought in after Brexit.

Some argue that the power-sharing structure no longer works in a changing Northern Ireland, where more than 40% of people reject the old sectarian labels and identify as neither nationalist nor unionist.

Catholics now outnumber Protestants for the first time, and the question of whether in the long run Northern Ireland will remain part of the U.K. or join the south — the issue that fueled the Troubles — remains unresolved. The Good Friday Agreement authorizes a referendum on Irish unification if polls ever indicate it would likely pass.

“It’s an imperfect peace in many regards,” Thompson said. ”(But) there are thousands of people that are probably spared injury and bereavement and imprisonment today because of the agreement.”

Olphert said his children, now in their 20s, grew up in a society transformed from the divided and dangerous place he once knew.

“They have no sense of what it was like, and I don’t ever want them to have a sense of what it was like, because it’s in the past,” he said. “The Troubles is now history to the generation of children growing up now. And that’s good.”