LOS ANGELES, United States (AFP)— A huge federal probe was underway Tuesday into what caused the deadly Los Angeles wildfires, with millions in the city clamoring for answers.

Social media has exploded with theories about what started blazes that tore through the city of Altadena and the upmarket neighbourhood of Pacific Palisades, killing at least 24 people and leaving whole communities in ruins.

Suggestions include downed power lines, deliberate arson, a stray firework and the reignition of an earlier fire.

But Jose Medina of the federal Bureau of Alcohol Tobacco and Firearms (ATF), which is leading the inquiry, said it was too early to say.

“We know everyone wants answers, and the community deserves answers. ATF will give you those answers, but it will be once we complete a thorough investigation,” he told reporters.

The ATF is working with local law enforcement, as well as the Forest Service and the US Attorney’s office, in an operation that will involve around 75 people.

Fire investigators, chemists, electrical engineers and sniffer dogs trained to detect accelerant will be doing painstaking fieldwork to find the seats of the two fires, he said.

A team will also be deployed to gather clues from the local community and online, conducting interviews with possible witnesses.

“We are following all the leads and processing all the physical evidence,” Medina said.

“ATF is determined to leverage every available resource to deliver a thorough and transparent investigation.”

Internet users have leapt on a video posted by trail runners that shows them running away from smoke in hills above Pacific Palisades.

But one of the men, Beni Oren, told the Los Angeles Times they had nothing to do with the fire, and had actually been fleeing for their lives in the video.

“It’s definitely kind of infuriating that people are blaming us,” he told the paper.

“Just knowing as a matter of fact… that we didn’t do it but then seeing the amount of people that have different theories is overwhelming.”

Local media reported that a number of homeowners in the Altadena area have launched a lawsuit against power company Southern California Edison after a video appeared to show flames at the base of an electrical transmission tower.

The utility has said it does not believe its equipment was at fault.