Arson is one of the most difficult crimes to prove because fire often destroys evidence. But these investigations are fascinating for the public.There are a lot of moving parts there because its a very difficult crime. Our evidence is sometimes compromised by the fire scene itself. Or damaged. Or leaves with the arsonist, i.e., the lighter in his pocket, said Reading Fire Chief Todd Owens. But people are curious about it because it doesnt happen with any real frequency.Owens is a former president of the Ohio Chapter of the International Association of Arson Investigators, and Special Operations Chair of the Hamilton County Arson Task Force, where he was also a past commander.He praised Cincinnati fire investigators for their quick work on the Big Mac Bridge fire, which resulted in four arrests this week.Cincinnati officials still can't say what started the fire that damaged the bridge so severely that it will be closed for repair until March.CFD Chief Frank McKinley said he can't release too many details about what caused the fire, how it started, or any reasoning as to why it was set because the investigation is still ongoing.But Owens said authorities dont need to prove a motive, or describe why arsonists set a fire, to win convictions. Its reckless at a minimum what theyre doing without regard for public safety. They had to know that their actions had an implication, Owens said. We really dont know what all their motives are. Sometimes we figure it out, sometimes we dont.Owens investigated a similar fire at Vorhees Park in 2012, where teenagers started a fire in a garbage can that spread to the roof of a picnic shelter.That small trash can fire progressed to a fully involved structure fire in about 15 minutes, Owens said. It was difficult to put out because the material on the ground was like pools of gasoline that melted down.Reading police and fire investigators solved that case by talking to witnesses and bystanders at the fire.When arson investigators arrive at a fire scene, they first talk to whoever is there, take their statements, and gather video and photo evidence. They next talk to the firefighters who fought the blaze. Did you have difficulty extinguishing the fire, did the fire get bigger when you applied water, did it spread out when you applied water, which could be indicative of the spreading of an accelerant that was already on the ground, Owens said. Because they have a great insight, they were there, they saw the fire while it was actively burning, and so they can provide us some key information initially that could help us determine the origin area of the fire and dial us in to really work to determine the cause of that fire.Fire investigators use a systemic approach to find a fires point of origin and eliminate potential ignition sources, said Nathan Bromen, an adjunct professor of fire science at the University of Cincinnati and a retired fire chief who now works for a private engineering firm as a fire investigator.Investigators move from the least burned area to the most damaged part of a fire scene taking photos and using drones to find where the fire originated, Owens said.Theres almost always some evidence that remains after a fire. People have a misunderstanding that fire consumes all of the evidence and that is not the case, Bromen said. All fires start small. The question is how much time does it have to spread and what are the combustibles that allow it to spread?Accelerants such as butane in a lighter, gasoline, or mineral spirits can start a fire. To determine this, investigators use specially trained dogs (which have 300 times the smelling capacity as humans) to smell for accelerants both at the fire scene and by sniffing the clothes of bystanders, Owens and Bromen said.The plastic and rubber at the 1000 Hands Playground underneath the bridge, combined with any trees or brush, is great fuel for the fire, Bromen said.Thats a pretty substantial fuel load that is there and that can contribute significantly to the fire spread, Bromen said.Cincinnati Fire Chief Frank McKinley said Wednesday that investigators used trained dogs to help determine the cause of the fire that damaged the Big Mac Bridge, as well as lab testing and boots-on-the-ground work.They used a lot of video, they used a lot of phone records, they used license plate readers and things of that nature, McKinley said. Thats what actually led us to our first arrest.Investigators cannot reveal any details about their case until arrests are made, Owens said.One thing that I think the public doesnt really understand is that any little bit of information that they release could negatively impact their investigation," Owens said. Only the people that set the fire know all those nuances.