A full 10 minutes later it was still going strong:
From the LA Times:
A massive explosion set off by fireworks at an Ontario house rocked a residential neighborhood Tuesday, prompting a large response from firefighters and law enforcement.
The city tweeted that a large cache of fireworks ignited at a house, setting the residence on fire. Smoke could be seen from as far away as Anaheim.
Two structures on the same lot — a front and back house — were engulfed in flames following the explosion, Ontario spokesman Dan Bell said.
“The roof is gone, burned out,” Bell said of the rear property. “The one on the front — I was on the street; I could see flames coming through the roof. It will most likely be a total loss.”
A horse, smudged with black soot, was seen trotting around the property after firefighters freed it from one building’s wreckage, according to Bell and video from ABC-TV Channel 7.
Apparently more than 50 firefighters were on scene, where obviously they have to be extremely cautious of further explosions. Here they are braving the flames to free that horse:
And here's a few more clips from bystanders that show the intensity:
At first I wasn't seeing any reports of fatalities but unfortunately as I was finishing up this post I saw word that two people on the property lost their lives. Just awful. Looking at the aftermath it's hard to imagine anyone in close proximity could survive. I have to wonder what on Earth so many commercial-grade fireworks were doing in a tightly-packed neighborhood, but most importantly am hoping all the first responders & neighbors are alright.