Billingsley told the inquiry that the PSNI would agree that the RUC, which was the police force in Northern Ireland at the time, had an “under-resourced” Criminal Investigation Department (CID) when the bombing occurred.

However, he added, the team set up by the RUC to investigate the bombing was “significant”.

“Certainly, at the time probably the largest and would probably still be significantly large today.”

He said the RUC received four to five boxes of phone data from network providers and work was carried out to identify calls linked to the bombing.

When asked by an inquiry lawyer whether one person had to analyse 500 million calls, Billingsley agreed this was the case.

He went on to say that, seven weeks after the bombing, a decision was taken to remove more than half the investigators from the team.

Det Ch Insp Billingsley said while he couldn’t state the “rationale” behind that decision, it was natural that any large inquiry would at some point “have to start scaling back resources”.

That went further between 2008 and 2016 when there was no senior investigating officer assigned to the bombing inquiry at the PSNI and therefore no “active investigation” occurred.