New satellite images obtained by CNN show recent activity at a previously undeclared North Korean facility that researchers suspect is being used to build nuclear warheads.

The imagery, captured by Planet Labs and analyzed by experts at the Middlebury Institute of International Studies, indicates that the facility, which is in the village of Wollo-ri near the country’s capital city of Pyongyang and has not been previously disclosed to the public, is believed to be linked to North Korea’s nuclear program and remains active.

“It has all the signatures of a North Korean nuclear facility — security perimeter, on-site housing, monuments to unpublicized leadership visits, and an underground facility. And it sits right next to a bottled water factory that has none of those characteristics,” Jeffrey Lewis, a professor at the Middlebury Institute of International Studies, which specializes in open-source intelligence, told CNN prior to publishing his own report.

“The big thing that sticks out is all the vehicle traffic — cars, trucks, shipping containers. This factory is very active. That activity has not slowed down — not during negotiations and not now. It’s still making nuclear weapons,” he told CNN.