As ATF investigators scoured the site, fire industry experts said bringing in federal agents ultimately means added resources, more experience and extra tools to help uncover a cause.
"I think it's just a good, smart move to call in ATF, FBI, whoever you want when you have a fire this significant," Retired Menlo Park Fire District Chief Harold Schapelhouman told ABC7 News.
With decades dedicated to the fire service, Schapelhouman described the fire as aggressive and unusual for a big box store.
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"Aggressive, angry. I mean, there's a lot of words to kind of describe when you watch that column coming out of there, with dark black smoke. But you know, you have a facility that has a lot of vertical storage and racks, and a lot of combustible products, right? Everything from propane cylinders, to roofing materials to, you know, adhesives, to lumber," he added.
He anticipates they will have to stabilize walls, clear paths and more- preparation needed for the long investigation ahead.
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"Sometimes it's a bucket at a time that you're taking out of a building like this- of soot and ash and debris- to try and dig down to where you think the point of origin was, and determine cause," he continued. "So you know... How long does all that take? As long as it takes."
"They'll be very much inundated, working 12 hours," Stan Fernandez said. "Eight to 12 hour shifts."
Fernandez is an adjunct professor for City College of San Francisco's Fire Technology Program.
Echoing Schapelhouman's support for the assistance of ATF agents, Fernandez shared, "They understand fire behavior, evidence collection, forensics, the magnitude of different types of fires."
"Not only fire, but explosions," he added. "They come from all different denominations of specialty- electrical, structural. They do scene processes, interviews and all that."
When asked about the Home Depot site itself, Fernandez told ABC7 News the big challenge is going to be establishing safety with the roof gone.
VIDEO: Massive fire tears through Home Depot in SJ
Inside the store, he described what would be considered in the investigation, "There's the heavy fire load- any box store will have a heavy fire load. You're looking at the distribution of product, the aisle space, where it was in relative to where the initial start of the fire, how it did- how it moved. There things that take into consideration of ventilation, the ventilation factor."
Fernandez explained the Home Depot fire investigation is sure to span far beyond the physical fire scene.
"They have to gather a lot of data," he said. "When I say data, I mean it comes from a myriad of areas. It comes from the people from interviews, eyewitnesses, cameras inside the store, outside the store, passersby... they have to interview those people."
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The ATF and San Jose Fire have asked that people who were at the Home Depot when the fire started, come forward and give their account.
Witnesses who spoke with ABC7 News claimed there was no immediate fire alarm and no sign of active sprinklers. The fire department said that is all under investigation.
"In this case, if in fact true, not having any suppression within the sprinkler systems... It's like, what happened? Was it overwhelmed? Was it not enough? Or was it shut off? I think those are the things that the fire department- now with ATF help- are gonna come out and present at some point," Schapelhouman shared.
He added, "You can overwhelm sprinkler systems- and we've seen that before in large warehouse facilities- meaning that the fire gets so big, the ability of the sprinkler system is not there to be able to put it out. So that's a design issue, that's a combustible load issue, that's a code issue. And those are all real factors."
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When that time comes, he said what is ultimately identified could lead to widespread improvements.
"It's not as if that's the only Home Depot in the country, right?" he added. "Nobody's looking to duplicate this."
Back to the addition of ATF agents in the fire investigation, Schapelhouman shared, "I think it's a masterful move, whoever did it. They got beat up many years ago on the Santana Row fire for not requesting ATF. So, I don't know if that was a factor in this case, I hope that it wasn't. I think it's just a good smart move."