Maxar Technologies released satellite imagery on Sunday showing activity at Iran's Fordo nuclear facility prior to U.S. air strikes.

Iran’s most heavily fortified nuclear site is at Fordo, a hilly area south of Tehran and about 15 miles away from Qum, a city where more than a million people live.

Fordo is Iran’s second nuclear enrichment facility after Natanz, its main facility, which already has been targeted by Israeli airstrikes. The IAEA says it believes those strikes have had “direct impacts” on the facility’s underground centrifuge halls.