Orange County fire officials said on Monday that the threat of a major explosion from an unstable tank of toxic chemical was eliminated, but that danger of a smaller scale explosion remained.

“The crisis is not averted,” TJ McGovern, the interim fire chief for the Orange County Fire Authority, said at a news conference on Monday. “Let me be clear. Evacuation zones are still in place. The threat is not eliminated.”

In a separate video interview with the TV station KTLA, Mr. McGovern said that the tank could still leak or explode, though not to the extent they first feared.

That is why, he said, evacuation orders remained in place for more than 40,000 residents who live near the industrial site in Garden Grove, Calif., as local authorities assessed whether it was safe for residents to return home.

The statements on Monday came after the authorities said that a crack had continued to relieve pressure in the tank and that temperatures inside had dropped. On Sunday night, they said, firefighters — who have been hosing down the 22-year-old tank for days in an attempt to cool it — climbed onto the equipment and removed insulation to hasten their progress.

“The trajectory is headed in the right direction for the first time since Thursday,” Thomas J. Umberg, a state senator who represents the area that has been evacuated, said Monday.

“The challenge now is that people want to get back in their homes,” he said, adding “and they can’t go back until it’s safe.”

Firefighters responded on Thursday to an industrial site, belonging to GKN Aerospace, where a tank containing about 7,000 gallons of the chemical methyl methacrylate was becoming pressurized and releasing gas as the tank overheated.

According to the Environmental Protection Agency, exposure to methyl methacrylate can irritate the eyes and skin and make it difficult to breathe, among other symptoms. Birth defects have appeared in exposed animals.

Local authorities issued evacuation orders to thousands on Friday, as the tank’s temperature increased and the tank itself bulged, a sign that it could potentially explode. On Sunday, officials said they found that the tank containing the highly flammable chemical had cracked, relieving some of the internal pressure. Internal temperatures also dropped from 100 degrees to 93 degrees.

“The crack is there,” Craig Covey, an incident commander with the fire authority, said on Monday. “We have verified that it’s there, and the tank has released its pressure.”

The Orange County District Attorney’s Office said on Monday that it was investigating GKN, a British manufacturing company that produces jet engine parts, landing gear and other components for military and civilian aircraft.