The head of the Russian-occupied Zaporizhzhia nuclear power plant in Ukraine has been released, U.N. nuclear watchdog chief Rafael Grossi said on Monday after a detention that Ukraine blamed on Russia and called an act of terror.

Ukraine said a Russian patrol detained Ihor Murashov on Friday as he traveled from Europe’s largest nuclear power plant to the town of Enerhodar, where many of the plant’s staff live. Ukrainian staff continue to operate the plant in conditions the International Atomic Energy Agency says put safety at risk.