In September, South Korean President Moon Jae-in said that North Korean dictator Kim Jong Un had promised to "permanently dismantle" the Tongchang-ri site "in the presence of experts from relevant nations."
Recent construction at the site reportedly includes rebuilding a launch pad and a missile engine test stand. Beyond Parallel said that the construction at the site was "deliberate and purposeful."
The groups said that changes in the site could be seen between Feb. 16 and March 2, although it's not clear when they occurred within this time frame. Because of this, it's impossible for them to tell if the construction began before, during, or after President Donald Trump's summit with Kim in Hanoi, Vietnam, on Feb. 28.
The New York Times reported that officials from South Korea's National Intelligence Service thought that the work at the Tongchang-ri site began before the summit took place. A South Korean lawmaker confirmed to CNN that the National Intelligence Service believed that the Tongchang-ri site was being restored.
The website, 38 North, was founded by two North Korea experts from Johns Hopkins University. It has released analysis of previous satellite images before. Both 38 North and the Center for Strategic Studies' Beyond Parallel project are considered to be respected sources for analysis on North Korea, CNN said.
There are also reports of activity at the Yongbyon nuclear complex, CNN reported. Dismantling this complex in particular had been a bargaining chip that Kim had reportedly offered during the Hanoi summit.
The CIA has so far refused to confirm these reports.
The summit between Trump and Kim in Hanoi broke down on Feb. 28 after Kim reportedly demanded that all sanctions on North Korea be lifted.