Satellite imagery of a military training complex near Pyongyang indicates that North Korea may have used heavy anti-aircraft machine guns in a public execution last October, experts on the country's human rights situation and military wrote in a report on Wednesday.

"Sometime on or about October 7th, 2014, some very unusual activity was noted on satellite imagery of the Kanggon small arms firing range," wrote Greg Scarlatoiu, executive director of the Committee for Human Rights in North Korea and veteran North Korea military expert Joseph Bermudez Jr., of AllSource Analysis, Inc.

Kanggon Military Training Area lies 22 km (13 miles) north of the capital city Pyongyang, near an elite military academy, and is likely used by military academy students, military units and state security authorities, the report said.

On last Oct. 7, the authors said, "instead of troops occupying the firing positions on the range there was a battery of six ZPU-4 anti-aircraft guns lined up between the firing positions and the range control/viewing gallery."

The ZPU-4 is an anti-aircraft gun system consisting of four 14.5mm heavy machine guns (similar to a U.S. .50 caliber heavy machine gun) mounted on a towed wheeled chassis, the authors wrote.

The satellite imagery, which the authors say appears to have been taken just before the executions, also shows either a line of troops or equipment, five trucks, a large trailer, and a bus.

Public execution most plausible

"This suggests that senior officers or VIPs may have come to observe whatever activity was taking place. Most unusual in the image, perhaps, is what appears to be some sort of targets located only 30 meters downrange of the ZPU-4s," the report said.

Using such heavy guns on the small arms firing range "is neither safe nor practical" because it would destroy the backstop downrange, the report said. Likewise, a live-fire test of the weapon on that range would be "even more nonsensical" because the ZPU-4 fires rounds with a range of 8,000 meters (yards) and can reach a maximum altitude of 5,000 meters, the authors wrote.

"Busing in senior officers or VIPs to observe a ZPU-4 dry-fire training exercise at a small arms range amidst North Korea’s fuel shortages would make no sense," they added.

"The most plausible explanation of the scene captured in the October 7th satellite image is a gruesome public execution," said the report, which said 24 heavy machine guns would leave the bodies of those shot "nearly pulverized."

"The gut-wrenching viciousness of such an act would make 'cruel and unusual punishment' sound like a gross understatement," the authors wrote.

"Given reports of past executions this is tragic, but unfortunately plausible in the twisted world of Kim Jong-un’s North Korea," they add.

The report by Scarlatoiu and Bermudez comes on the heels of reports by domestic and foreign media in Seoul quoting lawmakers briefed by South Korean National Intelligence Service (NIS) chief Lee Byoung Ho as saying North Korean leader Kim Jong Un has had 15 senior officials executed so far this year.

The Associated Press reported that Lee told the lawmakers in a closed-door hearing that a North Korean official with a rank comparable to a vice Cabinet minister in the South was executed in January for questioning Kim's policies on forestation, while another official of similar rank was executed in February for opposing Kim's plans to build a giant building in the shape of the Kimilsungia flower, named after his grandfather, North Korea founder Kim Il Sung.

The NIS chief also told the lawmakers that the agency also believes that North Korea used a firing squad in March to execute four senior members of the Unhasu Orchestra, a Pyongyang music and performing arts troupe.