Calmer winds helped firefighters gain ground on Saturday against fires that have destroyed homes and raced through nearly 20,000 acres of northern and eastern San Diego County brush land.

Authorities charged a man for adding fuel to one of the nearly dozen blazes.

A new fire at the Camp Pendleton Marine base left some evacuations in place.

Thousands of firefighters and fleets of water-dropping military and civilian helicopters planned fresh battles on Saturday as investigators continued to seek the causes of the fires that burned at least eight homes and an 18-unit condominium complex, emptied neighbourhoods and spread fields of flame, smoke and ash that dirtied the air in neighbouring Orange County and as far north as Los Angeles County.

Alberto Serrato, 57, pleaded not guilty on Friday to an arson charge in connection with one of the smaller fires, a 105-acre fire in suburban Oceanside that started on Wednesday and is now fully contained.

Tanya Sierra, a spokeswoman for the San Diego County district attorney's office, said witnesses saw Serrato adding dead brush to smouldering bushes, which flamed up. He has not been connected to any other fire, Sierra said.

Oceanside police Lieutenant Sean Marshand said Serrato was believed to have added fuel to the fire but not to have started it.

"Unfortunately we don't have the guy that we really want," he said.

Serrato remained jailed on Friday; Sierra said she didn't know whether he had an attorney.

All together, the wildfires about 30 miles north of San Diego have caused more than $20m in damage.

Three fires continued to burn at Pendleton: a 15,000-acre blaze that began on Thursday was 40% contained, and a new fire on Friday that quickly grew to 800 acres was 25% surrounded that night. A 6,500-acre fire that started Wednesday at a neighbouring navy weapons station and rolled on to the base and the city of Fallbrook was 65% contained.

At their peak, the fires prompted about 8,400 military personnel and their families to be sent home from parts of the sprawling coastal base between Los Angeles and San Diego, but some housing-area evacuations were lifted, base spokesman Jeff Nyhart said.

The most destructive fires started in Carlsbad – a densely populated coastal suburb of 110,000 people where a badly burned body was found Thursday in a transient camp – and San Marcos, a neighbouring suburb of 85,000 people where strip malls and large housing tracts mix with older homes whose residents cherish their large lots and country living.

The Cocos Fire, which hopscotched through San Marcos and Escondido, was 70% contained Saturday morning after burning 2,520 acres, the county reported.

As some evacuations were lifted, residents returned to their homes not sure what they would find.

"We thought for sure it was gone," said Lauren Frost, 31, whose family had left their Escondido home for the second time in two days on Thursday and watched on television as flames burned across the street.

The Frost house survived, but two were reduced to rubble on Mount Whitney Street, about 30 miles north of San Diego.

The region had become a tinder box in recent days because of conditions not normally seen until late summer – extremely dry weather, 50mph Santa Ana winds and temperatures in the 90s fahrenheit. On Friday, slightly cooler weather and calming winds aided the 2,600 firefighters, and thousands of people began returning home.

Al Said of Escondido refused to evacuate and helped firefighters save his home with a garden hose. Two of his neighbours lost theirs.

"That house burned and the house next to it burned," he said. "By the grace of God and the hard work of these firefighters, they came in and they saved my house right here."

He's happy his home survived, "but yet I look at my neighbour's property and what do you say? Just devastating," he said.

Eight of the San Diego County blazes popped up between late morning and sundown on Wednesday, raising suspicions that some had been set.

In Carlsbad, investigators finished examining the burn site across the street from a park and focused on interviewing people who called a hotline that was set up to report any suspicious activity.

"Do people have suspicions? Yes," said police Captain Neil Gallucci, noting there has been no lightning that could explain the blazes. "But can we confirm them? The answer is no."

Police in Escondido arrested two people, ages 17 and 19, for investigation of arson in connection with two small fires that were extinguished within minutes. But they found no evidence linking the suspects to the 10 biggest wildfires.

The Bernardo fire, the first of the North County blazes to break out, burned 1,548 acres and was 95% contained by Friday night.

A backhoe operator at a development site accidentally started the fire while digging trenches, San Diego fire officials said on Friday.