An inadequate number of detectives was available to investigate the Kingsmill killings, a retired officer has said.
Less than 25 were deployed at any one time to probe the deaths of 10 Protestant workmen in South Armagh, former Royal Ulster Constabulary (RUC) detective chief inspector James Mitchell said.
Mr Mitchell's thinly-spread team of investigators covered South Armagh and South Down, one of the most dangerous areas for the security forces to operate, where support for the IRA was at its strongest.
He told a Belfast inquest: "It was an abnormal situation in South Armagh, the manpower was totally inadequate.
"You had detectives coming from Belfast who had no idea of the geography of South Armagh.
"You had added problems on top of the actual manpower, the logistics of running an investigation with such inadequate manpower and with the security level in South Armagh."
His group of 25 detectives, boosted after the Kingsmill attack, had to investigate ordinary as well as paramilitary crime and the former detective said he only had around a dozen available at one time.
More than 1,000 officers are working on the Manchester bombing at present, a barrister pointed out.
Mr Mitchell said it would have been helpful if some of the on the runs (OTRs) from justice who have been suspected of involvement had been living in Northern Ireland rather than the Republic, so they could have been arrested by the RUC.
He added: "It would have been helpful if we had been in a position to have more resources in the operation and it would have been helpful if we had had better and more accurate intelligence."
Last week, the inquest was told two senior IRA members were responsible for the massacre.
Brian Keenan and Seamus Twomey were named in court by one of their former comrades, police informer Sean O'Callaghan, as the men behind the atrocity.