"These are going to be placed in the bedroom, probably three or four different places around the bed, and possibly in the hallway, the idea that these people had to get out of bed and to the front door … we will be able to get a better understanding of the conditions they were exposed to during their egress from the house," Grove said of the devices.
The sensors were placed throughout the rooms to record temperatures at each second of the fire.
"This equipment allows us to measure gas temperatures, flame temperatures," Grove explained. "We will get a vertical profile, all the way from the ceiling down to the floor."
ATF investigators also mounted cameras, to document what happens at every angle -- another tool to gather information from the fire.
"We have four camera places, two outside, two inside the structure, and we will take still photographs," said Grove. "It's one thing for an engineer to get on the stand and explain to a jury and try to talk science to a jury. It's another thing to show them a video of what happened."
With a few finishing touches, it was almost time. The structure, re-created with furniture as close as possible to the original home, was ready to burn.
"When we set this fire, we are going to get certain temperatures that we measure, certain heat fluxes. We will never say they are exactly what happened at the night of the fire, but they will be similar," Grove said.
So on June 12 of last year, with computers, cameras and sensors tested and ready to go, and firemen standing by, the test began.
The fire was ignited using an amount of gas the investigators believe is less than that used by the suspect. The flames grew quickly, attacking everything in reach.
Agents manned their stations, studying every reading, while computers documented it all.
The bedroom quickly became hell, reaching 800 degrees in 90 seconds.
Again and again the rooms were burned, and each time, the ATF said the results were conclusive.
"We know looking at a fire like this, and a jury would understand, that if they don't get out of that space in a matter of seconds they are not going to make it, or they are going to be severely burned, which they were," Grove said.
The evidence was mounting that Jason Henry poured gas throughout the home, including in his parents' bedroom, in a manner that prosecutors said could have no other result than severe injury or death.
"There really aren't words to describe what they had gone through in the last moments of their life," Dalton told ABC News. "I don't think anyone can imagine the horror of waking up to flames surrounding your bed, on your bed, and realizing that your chances for survival are nil."
"Mrs. Henry indicated, according to Jason, that she was yelling, 'Oh my God, oh my God.' And for her to have to go through that, in her last moments of her life, after having leukemia, and dealing with that painful disease for many years and having her life end at the hands of her son ... is just beyond my comprehension."
Police say Jason Henry was no stranger to playing with fire having found that he and friends had made videos of themselves setting fires.