Mendocino Complex Fire now 2nd largest wildfire in California history

Twin wildfires fueled by dry vegetation and hot, windy weather have continued to grow in Northern California, making the Mendocino Complex Fire the second largest wildfire in California history.

Only the Thomas Fire that started last December in Ventura and Barbara counties is larger by acreage, according to CalFire's top 20 California wildfire list. 

California fire officials said Monday that the two fires about 100 miles north of San Francisco were 30 percent contained and have scorched 428 square miles, or about 275,000 acres. Its cause is under investigation.

The Ranch and River  fires, which span Lake and Mendocino counties, are burning about 14 miles apart and have destroyed 75 homes. Another 9,000 buildings are threatened.

The two fires cover an area larger than a deadly wildfire burning near Redding, California.

That blaze has killed two firefighters and five civilians and destroyed more than 1,000 homes.

The wildfire started two weeks ago by sparks from the steel wheel of a towed-trailer's flat tire. It is 45 percent contained.

The Associated Press contributed to this report.