The surprising images were taken on Sunday as tensions between the United States and North Korea seemed to spike. The Korean Peninsula pulsed with news that the North was preparing for its sixth atomic detonation and that American warships had been ordered into the Sea of Japan as a deterrent, even though the ships turned out to have sailed in the opposite direction.

The volleyball games, played in the middle of that international crisis, were probably intended to send a message, analysts said, as the North Koreans are aware that the nuclear test site is under intense scrutiny. But what meaning the North wanted the games to convey is unclear.

“It suggests that the facility might be going into a standby mode,” Joseph Bermudez told reporters on a conference call organized by 38 North, a research institute at the Johns Hopkins University School of Advanced International Studies. “It also suggests that these volleyball games are being conducted with the North Koreans knowing that we’ll be looking and reporting on it.”