Mr. Stevens eventually zeroed in on Brian Nelson, the undercover agent who smuggled the South African guns that wound up in the hands of loyalist paramilitaries and was also implicated in helping to set up Pat Finucane’s assassination. The Stevens team planned to arrest Mr. Nelson in 1990. But hours before the planned raid, Mr. Nelson fled to England, apparently tipped off. The same night, a mysterious fire broke out in Mr. Stevens’s office, which was inside Northern Irish police headquarters, destroying most of the office and its contents.

To this day, rights advocates say, the British government stalls investigations by saying it can’t find its former soldiers. In the ongoing inquest into the shooting deaths of three teenagers and a priest in Springhill in 1972, for example, civilians and witnesses have testified, but the investigation hangs in limbo because the Defense Ministry claims it can’t find the soldiers who opened fire.

“It’s a standard prevarication,” said Mark Thompson, who co-founded Relatives for Justice, an NGO, after his own brother was shot dead by members of the Force Resistance Unit. “They spend time, then they say the soldiers have PTSD or Alzheimer’s, and then it’s an application for anonymity or public interest. They drag it on for years and years.”

***

I met John Finucane on a drab summer day in Belfast, when rain ran in the gutters and pushed the crowds of shoppers and sightseers into doorways. Mr. Finucane’s law firm sits near the spot where the Falls Road, a rambling thoroughfare that has long been Belfast’s Catholic heartland, dotted with Irish cultural centers, I.R.A. memorials and repurposed linen mills, touches the flank of the city center.

With a politician’s optimism, Mr. Finucane pointed out that his own kids have never known their city as a war zone. They don’t get searched and questioned riding the bus downtown. Mr. Finucane won his seat in Parliament in 2019, wresting it from a long line of Protestant, unionist politicians, the last of whom held it for over 18 years.