Authorities in Orange county, California have ordered the evacuation of 40,000 people over concerns about a chemical leak that threatened to spill or explode.

The problem arose on Thursday at a facility owned by GKN Aerospace in the town of Garden Grove, where a storage tank holding methyl methacrylate began off-gassing and threatened to fail. The chemical, which is highly flammable, is used to fabricate resins and plastics.

Local authorities originally responded to the incident with a hazmat team on Thursday, ordering local residents to evacuate. They lifted the order later that day, but the problem worsened due to “damage to a valve on the tank” that “created additional operational challenges, preventing complete mitigation”, Garden Grove authorities wrote in an evacuation order.

By Friday, new evacuation orders had expanded to residents in six cities.

“We have a tank that is actively in crisis,” Orange county fire authority division chief Craig Covey told reporters at a news conference.

“There are literally two options left remaining. One: the tank fails and spills a total of about 6-7,000 gallons of very bad chemicals into the parking lot in that area. Or two: the tank goes into a thermal runaway and blows up, affecting the tanks that are around them that have fuel or the chemicals in them as well.”

Garden Grove police chief Amir El-Farra said some 15% of those facing evacuation orders were refusing to leave, according to the Orange County Register. The city has established at least two shelters for evacuees.

Covey, the county fire division chief, urged residents to take the order seriously.

“We are setting up these evacuations in preparation for these two options: it fails or it blows up,” Covey said. “Please follow our requests and orders for evacuations.”

It’s possible, however, that the tank could crack rather than explode. Emergency responders have laid out sand barriers to contain the chemicals in that event.

“In a weird world, that’s the best-case scenario, believe it or not,” Covey said, according to the New York Times.

GKN Aerospace is a division of a British corporation that produces airplane engines and other aircraft parts.