An estimated 40,000 people across California’s Orange County were under evacuation orders on Friday as officials warned a failing industrial tank holding thousands of gallons of toxic chemicals was set to burst or explode.

“We are setting up these evacuations in preparation for these two options: It fails, or it blows up,” Orange County Fire Authority Division Chief Craig Covey during a news conference Friday. “Please follow our requests and orders for evacuations.”

Crews began responding to the leak at an aerospace component manufacturing plant in the city of Garden Grove on Thursday afternoon. By Friday afternoon the leak had stopped, but problems including a faulty valve meant the tank was still an emergency risk.

The tank holds up to 7,000 gallons of methyl methacrylate, a volatile and flammable substance used to make plastics, officials said.

The crisis is unfolding at a facility for GKN Aerospace, which supplies components for commercial and military aircraft.

This is a breaking news story and will be updated with new information.