Sheriff's homicide detectives searching for the arsonist who set the deadly Station fire are appealing for anyone who travelled on the Angeles Crest Highway just above La Cañada Flintridge around 3:30 p.m. on August 25.

"Obviously if someone saw something that day we would like them to come forward and speak to investigators. But even if they didn't see anything we would also like them to contact detectives if they traveled past there around that time," said Steve Whitmore, sheriff's spokesman.

Detectives would like to be able to account for all the vehicles that passed the area near Mile Marker 29, where the blaze ignited.

The appeal comes as the Los Angeles County Board of Supervisors today approved an additional $50,000 reward for information leading to the conviction of the arsonist.

The state had already established a $100,000 reward in the fire, which has killed two firefighters and destroyed dozens of structures.

Investigators believe a substance found near the Station fire's point of origin helped spread the deadly blaze, and it has emerged as a key piece of evidence in the arson probe, a source told The Times.

The source would not identify the substance but said it was found in the brush off Angeles Crest Highway, walking distance from the turnoff that is at the center of the arson probe.

The source, who spoke on the condition of anonymity because the investigation is ongoing, said the substance was taken to a lab for testing to further verify arson investigators' findings. The recovered material is not a device, according to the source.

"There was material that didn't belong there. It was clear evidence that the fire was intentionally set," the source said. Sources said investigators are going through a database of arson suspects and fire bugs to search for possible suspects. The deadly Station fire is now 60% contained but winds have become a concern for firefighters.

Homicide investigators can be reached at (323) 890-5500.