Following the wave of wildfires that swept through Southern California this week, fire investigators are gathering clues left in the blackened ground.

By Friday, investigators had determined the causes of only a few of the 23 fires that ravaged swaths of seven counties from San Diego to Santa Barbara.

The 27,000-acre Santiago fire in Orange County was set by an arsonist, according to the Cal-Fire Web site. The 9,000-acre Rice fire in San Diego was caused by downed power lines, which are also the suspected cause of the 4,500-acre Malibu Canyon fire, said a Los Angeles County Fire Department spokesman.

The 2,800-acre Magic fire in Valencia began accidentally, with a spark from construction equipment, Los Angeles County officials said.

Determining a fire's cause can be a long process. To find out what caused a blaze, investigators must first learn where it started.

Eyewitness accounts often provide essential clues, but even without them, investigators gather information by reading the patterns on rocks, plants and scorched earth.

"The black scar that the fire leaves behind can actually tell a lot about how the fire was started in the first place," said Jessica Gardetto, a spokeswoman for the National Interagency Fire Center in Boise, Idaho.

Signs of an intense blaze

Wildfires gather intensity as they grow, often with the help of wind, Gardetto said. Because fires are hotter as they progress, their points of origin often have more vegetation left than spots burned later.

How and where plants are burned also provide information. A bush burned to a pile of white ash indicates an extremely intense blaze, while a patch of grass lying burnt on the ground shows the direction of a wind-driven fire's progress, she said.

Investigators usually know when lightning causes a fire because they have access to storm maps that show lightning strikes and rainfall amounts, Gardetto said.

Nationwide, lightning caused more than 16,000 fires in 2006, according to statistics compiled by the National Interagency Fire Center. The center reported more than 80,000 human-caused blazes.

Downed power lines are a common cause of wildfires in Southern California. Downed power lines were found near where the Malibu Canyon fire started, but investigators this week had not determined if they ignited the blaze or if the fire brought them down.

When the cause is human, more subtle clues are left. An investigator might come across the remains of a campfire, fireworks or a charred piece of metal thrown from a vehicle, Gardetto said.

Then there are the more unusual causes.

Within a few months in 2006, four fires in Idaho were touched off by birds that fell to the ground after being electrocuted when they straddled power lines, she said.

When a fire doesn't appear to be natural or accidental, investigators start looking into arson, said Rick Price, who recently retired from the Los Angeles City Fire Department after 34 years, 17 of them spent investigating fires.

Statistically, arson is not one of the more common causes of wildfires, but arsonists do tend to act when other wildfires are burning. When fires began breaking out in rapid succession last weekend, Price immediately thought some were arson.

Arson 'a big problem'

"When you have this many fires going on, for some reason it always happens," Price said. "It's not a big percentage, but it's a big problem."

The deliberately set Santiago fire destroyed 14 homes, injured four people and cost $4.5 million to fight as of Friday.

Sometimes investigators find a clue such as a matchbook that an arsonist used to start a fire, Price said. Otherwise the crime can be very hard to prove, because investigators must determine intent.

Investigators say there are several kinds of arsonists.

Some get sexual pleasure out of setting fires; others start blazes and then try to extinguish them in hopes of being recognized as heroes. Some do it out of spite.

"We see more of the revenge or the spite type — the quarreling lovers," said Blair Martin, an investigator for the Oxnard Fire Department. Oxnard firefighters deal mostly with fires in urban areas. Trash bins are common targets.

Accidents and natural causes, including power poles downed by wind, are the most common causes of wildfires in Ventura County, said Capt. Barry Parker, a spokesman for the Ventura County Fire Department.

A transient's campfire sparked the 2006 Day fire, which burned 162,000 acres in the Los Padres National Forest, and people grinding a pipe on private property started the 240,000-acre Zaca fire this year in the national forest.