"System and human error" have been blamed for the failure to identify a palm print recovered from a van linked to the Kingsmill massacre, an inquest has heard.
Ten workmen were murdered in a mass shooting in south Armagh on January 5 1976, in an atrocity attributed to the Provisional IRA.
A palm print was recovered from a van which police believe was used by the gunmen in 1976. It was not successfully matched until 2016.
The palm print had been taken from the van by police forensic officers, and attempts were made to match it using police database systems in 2010 and 2014. but these were not successful.
Giving evidence to an inquest into the atrocity, Detective Chief Inspector Ian Harrison put this down to "system and human error".
In 2016, a match was found for the palm print.
"This was subsequently checked and confirmed as a positive match by three officers," Mr Harrison told the inquest.
This sparked a fresh police investigation. A 59-year-old man was arrested by police in Newry, Co Down. He was questioned and a file was sent to the Public Prosecution Service.
In February 2017, the PPS announced it would not be pursuing a case against the man due to insufficient evidence.
Mr Harrison, who led the new police investigation, read a section of the report he submitted to the PPS to the inquest.
It revealed some difficulty proving whether the van had been used by the gunmen, due to a lack of witnesses and a lack of firearms residue inside the van, which was likely to have transported 11 heavily armed men thought to have fired more than 100 rounds at the scene.
Mr Harrison said he believes it is "more likely than not" that the van was the one used by the terrorists, but there are no witness accounts that can place that van at the scene.
There is also a lack of certainty over how the palm print came to be in the van.
Mr Harrison added: "We were not able to discount that the palm print had been placed there innocently."
The man who was arrested, referred to in court as S54, was asked to account for how his palm print was found in the van. He made no comment during police interviews.
Alan Black, the sole survivor of the atrocity, was in the public gallery with a number of the families of the 10 who were killed.
The hearing continues on Wednesday afternoon.