tterson, spokesman for the San Bernardino County Sheriff's Department. "We know that arson has already killed four people in our county. I think it's a strong possibility that the others are arson as well.''

In San Bernardino, witnesses told authorities that they had seen two men start the Old Fire on Saturday. Eight of the other nine fires are assumed to be arson until it can be proved otherwise.

Andrea E. Tuttle, director of the California Department of Forestry and Fire Protection said that her agency had not ruled out arson in any of the 10 fires that together have scorched more than a half-million acres and caused at least 16 deaths.

At the same time that thousands of firefighters were still trying to contain the blazes, arson investigators were already going through the ashes to try to determine the cause of the fires. They were seeking footprints, fingerprints, evidence of flammable liquid -- anything to focus the case on arson. "All of these investigations are still under way,'' Tuttle said. "We investigate every lead that we get.''

The Cedar Fire, which at 206,000 acres is the largest blaze, was apparently started by a 33-year-old hunter from Covina, Sergio Martinez, who told investigators that he had been lost and was trying to draw the attention of rescue helicopters. Martinez was issued a misdemeanor citation Saturday night by the U.S. Forest Service for setting an unauthorized fire.

"He did this because he was lost, allegedly, and was trying to locate his friend," San Diego County Sheriff Bill Kolender said Tuesday. "He thought by starting the fire his friend would be able to see him. Not a very good move." But authorities so far have refused to rule out arson even in the case of Martinez, according to Susan Please spokeswoman for the San Diego County Sheriff's Department.

"We are not ruling out arson until we have completed our investigation,'' Please said Tuesday night. "With all of the damage this fire is causing, we are proceeding very carefully in the criminal investigation.''

Authorities also established a hot line at (866) 346-7632 for tips about any suspected arsonists in any of the 10 fires.

Investigators need tips because despite huge advances in the science of collecting arson information, most arson cases go unsolved, according to Department of Justice statistics.

Many of the high-profile cases that do get solved are cracked as result of investigators focusing on specific suspects -- who often confess or give incriminating statements. Arsonists sometimes set structure fires for financial gain, but forest fires are generally set for more complicated reasons. In other cases, arsonists have admitted to setting fires for gratification.

A man convicted in 1997 of setting seven fires in San Diego County said he just wanted to be hero. A woman admitted that she had set the largest forest fire in Colorado history last year because she was depressed after breaking up with her boyfriend. Eight volunteer firefighters were arrested in Virginia in 2000 for setting several fires because they were bored.

A 2002 study by the Western Fire Ecology Center in Eugene, Ore. said that of 116,000 wildfires nationwide in the past decade, 25,000 were confirmed arsons, and another 25,000 were suspicious fires that might also have been intentionally set. The study also found that many convicted arsonists admitted to setting multiple fires.

"This could be the work of one person -- a serial arsonist,'' Patterson said. "But more likely it's copycats, people who see the media coverage and get ideas."

San Bernardino investigators released a composite sketch of one of two men seen throwing a burning object into the brush near Old Waterman Road and Highway 18. The two men, believed to be in their early 20s, were driving a gray 2000 or 2001 Chevrolet or Dodge van.

"There were no homes in the immediate area of the fire, but within 90 minutes there were homes burning,'' Patterson said. "We don't know their motive for this crime, but it is reasonable to suggest they got some kind of sick thrill out of this.''

The fire has destroyed 450 homes and caused thousands of people to flee the San Bernardino Mountains. Four elderly men died as result of heart attacks suffered from the stress of evacuation, investigators said.

"We consider this a quadruple homicide,'' Patterson said. "The persons who set these fires caused four people to die. Under state law that is murder. '' The four -- Charles Cunningham, 93, and James McDermith, 70, of San Bernardino, Gene Knowles, 75, of Big Bear and Chad Williams, 70, of Crestline -- all lived in areas popular with retirees. All four were forced to evacuate their homes on short notice, Patterson said.

The San Bernardino County Board of Supervisors authorized a $50,000 reward for the arrest and conviction of the person or persons responsible for starting the Old Fire.

The Los Angeles County Board of Supervisors also voted to offer a $50,000 rewards for information leading to the arrest and conviction of those responsible for two fires in different parts of the county.

Investigators said Tuesday night they did not know the cause of the Simi Fire, which has damaged a vast stretch of about 100,000 acres roughly bordered by Santa Clarita and Chatsworth in Los Angeles County and Moorpark and Simi Valley in Ventura County.

"Our arson unit is investigating the one in Chatsworth, but we have not actually determined it is arson fire,'' said Deputy Alba Yates of the Los Angeles County Sheriff's Department. "But we want to hear from the public. If you saw something -- if you know something, if you have any information about the cause of the fire, we really need your help.''

The Associated Press contributed to this report. / E-mail Jim Zamora at jzamora@sfchronicle.com

--------------------------------------------------------------------------------