One of the world’s biggest producers of aluminum has been hit by a serious ransomware attack that shut down its worldwide network, stopped or disrupted plants, and sent IT workers scrambling to return operations to normal.

Norsk Hydro of Norway said the malware first hit computers in the United States on Monday night. By Tuesday morning, the infection had spread to other parts of the company, which operates in 40 countries. Company officials responded by isolating plants to prevent further spreading. Some plants were temporarily stopped, while others, which had to be kept running continuously, were switched to manual mode when possible. The company’s 35,000 employees were instructed to keep computers turned off but were allowed to use phones and tablets to check email.