The population of North Korea’s city-size political prison camps could be tens of thousands lower than the estimate used for more than a decade by aid groups and the U.S. government, according to recent reports and accounts from researchers, who put the new number between 80,000 to 120,000.

Satellite technology allows improved ways of estimating one of the world’s hardest-to-pinpoint population figures, and the old range — 150,000 to 200,000 — could have been an overestimate, according to researchers.

But those researchers also say that the actual number of prisoners held by the North, not just the estimate made by outsiders, has probably fallen. They attribute the drop-off in part to a spate of prisoner releases at one camp, but they also say it is because the camps, in general, are so reliably lethal, killing faster than the pace at which people arrive. Some analysts also say the number of arrivals at camps has tapered off.

The drop-off comes amid several major changes, unexplained by the North, including the dismantling of two camps and the expansion of at least two other facilities — all in areas sealed off to visitors in mountainous corners of the secretive country.

The new population estimate was first released in January by the Korea Institute for National Unification (KINU), a government-funded think tank in Seoul whose researchers studied satellite images of the camps’ barracks. But the estimate has since gained wider acceptance among the small group of researchers in South Korea and the United States who study the North Korean gulag.

Last month, KINU’s research team testified about the likely prison population decline to a U.N. panel investigating rights abuses by the Pyongyang government. David Hawk, a research consultant for the U.S. Committee for Human Rights in North Korea, wrote in a separate report last month that the decline is “highly likely.”

Analysts caution that even basic information about North Korea’s prison camps is difficult to obtain or emerges years after the fact. The North denies the existence of the camps, and nearly all North Koreans are barred from technology that would allow communication beyond their borders. As a result, outside analysts tend to base research on satellite imagery and bite-size accounts from defectors.

“When we try to draw a big picture about these prison camp changes, we see holes,” Kim Soo-am, a researcher at KINU who co-wrote the institute’s report, said in an interview. “So we can only speculate. But one thing for sure is that the North is not paying more attention to human rights. Prison camps are still used to rule the country with horror.”

North Korean founder Kim Il Sung set up the camps, probably in the 1950s, as a way to purge his real and imagined political opponents. But the camps have since been used for a wider purpose, underpinning a system of unceasing surveillance and control. Those held at the camps, according to testimony from escapees, are given life-endangering labor assignments and starvation rations, which are cut further if work is not done well.

Analysts say that fewer people are now arriving at the camps. Under Kim, the North sent entire families to the gulag — not just the perpetrators, but also their parents and children. This practice has not stopped, defector testimony indicates, but it has slowed over the past 15 years.

The coinciding collapse of North Korea’s planned economy has also opened up new opportunities for bribery and corruption. “With a small bribe, one can escape many kinds of legal trouble,” Andrei Lankov, a professor at Kookmin University in Seoul, wrote recently in a column published on the NK News Web site.

The North once operated as many as 12 prison camps, but that number has dropped in recent decades, with the camps that remained growing larger. Analysts say it is unclear whether the latest closures — of camps 22 and 18 — indicate that the North is taking action in response to a declining prison population.

South Korean news outlets that employ defectors and maintain sources in the North reported last year that the North had shuttered Camp 22. Satellite images showed razed or abandoned guard towers and interrogation facilities. But it remains unclear what happened to the prisoners, estimated several years ago to number 30,000. Hawk’s report cited unconfirmed reports from defectors that 7,000 to 8,000 were transferred. Hawk cited another defector who reported a massive famine in the camp beginning in 2010 after poor harvests in the region.

“If even remotely accurate, this is an atrocity requiring much closer investigation,” he wrote.

The fate of Camp 18’s prisoners is slightly clearer, as prisoners there have been released in ­stages since the mid-1980s, with a handful escaping to the South. Defectors say that the last of those clearances was completed in 2006 and that the final several thousand prisoners were transferred to a nearby mountainous area near Camp 14, across the Taedong River. Experts say they are not sure whether Camp 18 has been folded into Camp 14 or exists as its own entity, albeit in a new area.

Depending on that distinction, North Korea’s gulag now consists of either four or five camps.