A new report has analysed S-21 prison documents – using a method normally reserved for so-called “big data” sets collected from social media – to shed light on internal purges as the Khmer Rouge tribunal closes its examination of the regime’s security centres.

The preliminary analysis, Emerging data sources and the study of genocide, found that while more than 12,200 people are recorded as having entered S-21, there were only about 8,200 complete records.

Of those people, nearly 63 per cent were executed within 2 months of arrest, but “as the regime continued, the rapidity in which prisoners were executed increased significantly. By 1978, two-thirds of prisoners were executed within 1 month of arrest; over 90% were executed within 2 months.”

There were three months that exhibited “exceptionally short” periods between arrest and execution: June 1976, when Kaing Geuk Eav assumed control of the centre; June 1977, when Hun Sen and other cadre fled to Vietnam amid purges; and December 1978, a period of purges before the regime’s fall.

Report author James Tyner said while more research was needed, applying methods pioneered during the social media age to research the decades-old S-21 data could help to “better understand the spatial and historical patterns [of violence]”.

He added it was possible to map “strings” of arrests – which spurred confessions of “traitorous” social networks and, in turn, more arrests – over time and space.

But Nuon Chea defence lawyer Victor Koppe maintained the study reveals nothing new and was “based on the assumption that anyone who entered S-21 didn’t leave it”, saying that his team believed evidence shows no more than 6,000 people had been executed at S-21.