About 50,000 people who were evacuated because of the threat of a failing chemical tank in Orange County, California, have now been allowed to return home.

The tank, which contains thousands of gallons of methyl methacrylate - a highly volatile and flammable substance used to make plastic - was recently deemed to be at risk of either a major spillage or an explosion.

But California fire officials later reassured locals, saying an operation had eliminated the risk from the chemical tank.

They added that testing had showed there was no chemical leak and no more risk of an explosion or to the public.

Response teams had been working to mitigate spill potential, and a "very small explosion concern", Orange County Fire Authority Interim Chief TJ McGovern said on Monday evening.

Earlier, Orange County Fire Authority Interim Chief TJ McGovern said the threat of a BLEVE, or Boiling Liquid Expanding Vapor Explosion, had been "eliminated" after an overnight operation from Sunday into Monday.

Although there was then no active leak, officials cited the risk of another leak as a reason for reduced evacuation to remain in place at that time.

A crack was found on the tank over the weekend, and officials became hopeful that this would allow pressure that had built up inside the tank to be gradually released.

Officials have now repeatedly said that no chemicals are currently leaking. They also say they have been working around the clock to ensure that it stays that way.

Emergency crews sprayed the tank with water to try to stabilise its temperature, following a series of rapid heat increases since Thursday.

Some 50,000 people were initially affected by precautionary evacuation orders, which included the cities of Garden Grove, Stanton, Anaheim, Cypress, Westminster, and Buena Park, and a state of emergency was declared by California Governor Gavin Newsom.

The tank at GKN Aerospace Transparency Systems, an aerospace facility, is in the city of Garden Grove, about 35 miles (56km) south-east of Los Angeles. GKN Aerospace has apologised to local residents.

At an earlier stage, officials expressed hope that emergency teams could relieve the pressure issue by slowing the rate of the chemical reactions that were causing the substances to heat up inside the tank.

"Letting this thing just fail and blow up is unacceptable to us," said Craig Covey, the Orange County Fire Authority division chief.

Officials had also been developing "very aggressive plans" for a scenario in which the tank's integrity fails, Covey said at the time. They have been creating dykes and dams to contain any chemicals if the tank does spill - with the aim of preventing any substances from reaching storm drains or the ocean.

Methyl methacrylate is used to make resins and plastics. The chemical is irritating to the skin, eyes, and mucous membranes in humans, and can cause respiratory and neurological symptoms with exposure, according to the Environmental Protection Agency.