REDDING, Calif. – The explosive wildfire that has forced the closure of a stretch of the Pacific Coast’s primary interstate in both directions swelled to 22,000 acres on Wednesday night but remained at zero containment.

Scores of homes and seasonal residences are being threatened by the Delta Fire, which is burning on both sides of Interstate 5 in Shasta County and pushing northwest into Trinity County. Some mandatory evacuations have been ordered.

The two counties were devastated earlier this summer by the Carr Fire that killed eight people, burned 360 square miles and destroyed more than 1,000 homes. The Carr Fire, fully contained last week, cost nearly $160 million to douse.

Insurance Commissioner Dave Jones said Thursday that insured residential and commercial losses from the Carr and the recent Mendocino Complex fires topped $845 million and are now counted among the most destructive wildfires in the state's history.

"Our wildfire history tells the story of how our fire season has changed over the years from a four-month season to a year-round threat," Jones said.

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"Life safety is a top priority for firefighters" combating the Delta Fire, the National Forest Service said in a statement. "Firefighters are working quickly to protect communities and structures immediately threatened by the fire."

The Delta Fire, which has spewed flames some 300 feet into the air, will "burn actively into the [Thursday] night," according to a 7 p.m. PST update on Inciweb. The 24-hour incident forecast said fire activity would "increase as a dry cold front approaches from the northwest with increased southwest winds."

Officials said the blaze was "human-caused" but didn't say whether it was arson or accidental.

There is no estimate on when several miles of I-5, the only continuous highway to touch the borders of Mexico and Canada, will reopen. Burned-out vehicles litter the freeway. Trees adjacent to the freeway also have burned, and officials must determine how many must be removed before the highway can be safely reopened.

U.S. Forest Service workers helped the driver of one burning truck to safety, and firefighters were among those who assisted other drivers. In a video, a passenger in one vehicle screams, “Oh my God, I want to go!” as trees burst into flames.

About 17 tractor-trailers were abandoned and at least four caught fire, Lt. Cmdr. Kyle Foster of the California Highway Patrol’s Mount Shasta office told the Los Angeles Times.

“There’s vehicles scattered all over,” Brandon Vaccaro with the California Department of Forestry and Fire Protection told the Redding Record-Searchlight, part of the USA TODAY NETWORK. “Whatever occurred here was probably pretty ugly for a while.”

Amtrak halted its Coast Starlight service between Sacramento, California, and Klamath Falls, Oregon. The company said in an email that it would resume when "conditions safely permit."

Contributing: The Associated Press