Apparently not. They seem to be going on what they saw on a surveillance video. They had dogs in sniffing for accelerants. See this story from KVUE, Austin
They also interviewed more witnesses and viewed dozens of hours of video surveillance tapes that helped them initially conclude arson is to blame for the weekend blaze, said state Fire Marshal Paul Maldonado.
...
Investigators believe one person was on the premises and set the fire, but they haven't ruled out that more people were involved, Maldonado said.
Dog team searches will help locate possible fire accelerants inside now that all fire hot spots have been extinguished, he said.
There were other stories on other sites that also indicated that they had not yet found any accelerants, as of mid morning on Monday.
If they've brought in a national team, they're not taking any chances. They may figure they have a good chance of catching the perp, and thus want to dot their 'i's and cross their 't's on the forensic evidence. The dogs are amazing, they can find traces of accelerants when the mechanical sniffers can't. I am very sensitive to mercaptans, and I'm better at finding a gas leak than the Atlanta Gas Light repairman's little machine . . . and my dogs' noses are many times more sensitive than mine.
Wish I'd had video to help nail an arsonist! Our old vain wish about "catching the guy with a match in his hand and a smile on his face" may be true for the very first time!
In our cases, the motives were almost always financial when arson was involved -- another old saying, "The mortgage rubs against the insurance policy and you get spontaneous combustion." This could well be politically motivated -- and they won't have any evidence in that regard until they catch their man.
A very, very strange case! It sure has knocked my 'conventional wisdom' into a cocked hat!
Have you heard anything about what they saw on the video?