Firefighters have been working through the weekend to cool a chemical tank “literally on the edge” of exploding in Southern California as they worked to avert a disastrous “worst-case” scenario, the incident’s commander said.

The 7,000-gallon tank containing methyl methacrylate, a toxic chemical used to manufacture resins and plastics, could also then set off an explosion in a 15,000-gallon tank nearby that holds the same substance, Chief Craig Covey, the Orange County Fire Authority incident commander, told NBC News on Saturday.

“We’re talking about possibly one of the worst chemical incidents in California history,” he said.

Gov. Gavin Newsom on Saturday declared a state of emergency in Orange County and directed his Office of Emergency Services and other state agencies to support affected residents.

The problems that have prompted officials to evacuate tens of thousands of people from Garden Grove, California, and from parts of neighboring cities include loss of valve control and rising heat inside the tank, which has already reached the volatile range of 90 to 100 degrees Fahrenheit, Covey said.

Those temperatures are a prelude to a “thermal runaway” that could trigger an explosion, he said.

“We are literally on the edge of it,” he added.

But officials said Sunday morning that a late-night operation provided firefighters with what they described as encouraging new information that could alter their strategy.

“Last night was a successful operation for this emerging incident and this ongoing incident,” interim Orange County Chief TJ McGovern said. Firefighters entered the area overnight and were able to visually inspect the tank, where they identified a potential crack that may be “relieving some of the pressure building inside.”

McGovern said crews were working Sunday to “vet and validate” the information, adding that the discovery “could change our trajectory and our strategy to this event.”

Firefighters have been using sprinklers and hose lines to cool the facilities. They hope that the process will trigger “curing” in the chemical that would further slow its warming enough to thwart a blast.

Covey said there’s a possibility for a “worst-case” scenario in which the three tanks on site — including a 4,500-gallon tank — all explode.

The hazmat events started Thursday with a “vapor release” at the aerospace manufacturing facility’s 7,000-gallon tank, Orange County fire officialssaid in a statement.

First responders at GKN Aerospace may have initially measured faulty temperature readings because they were taken from outside the tanks’ 1-inch-thick steel walls, Covey said. On Saturday morning, some fire personnel got perilously close to the stored chemicals and discovered they could read an internal temperature far greater than any they had seen, he said.

The new assessment measured tank temperatures rising from 77 to 90, with an average increase of 1 degree per hour Saturday afternoon, Covey said.

“We’re doing everything we can to find ways to mitigate that,” he said. “The worst-case scenario is that fireball.”

Even if a blast is averted, officials worry the tank could rupture and the chemical could leak into nearby storm drains and make its way to the Pacific Ocean. Covey said officials are working on contingency plans, including the possibility of preventing some of the chemical from getting into the storm drains, as well as efforts to “dike, dam and divert it” if it does.

“This is serious,” Covey said. “This is real.”

Orange County health officer Dr. Regina Chinsio-Kwong said those outside the evacuation zone are unlikely to face health risks, but she warned the incident “is a unique situation.”

“We don’t have information of a similar situation where this happened,” she said at a news conference Friday.

Evacuation orders have been issued for Garden Grove, about 35 miles southeast of downtown Los Angeles, as well as neighboring cities Cypress, Stanton, Anaheim, Buena Park and Westminster. Garden Grove Police Chief Amir El-Farra said Friday that the orders cover roughly 40,000 residents.

Newsom’s emergency declaration unlocks additional state resources, such as state-owned properties and fairgrounds that might be available as shelter for evacuees, the governor’s office said.