“The guys are beating their brains out,” she said, and she was frustrated. “And so am I. If I try to predict how much energy would be needed in that room for a flashover condition, the fuel load isn’t there. According to Sparkes, there was nothing but a throw rug and towels. Cabinets and fixtures were customized brushed steel. The shower had a glass door, the window had sheer curtains.”

She paused as the coffeemaker gurgled.

“So what are we talking about?” she went on. “Five, six hundred kilowatts total for a ten-by-fifteen-foot room? Clearly, there are other variables. Such as how much air was flowing through the doorway . . .”

“What about the rest of the house? You just said there was a big fuel load there, right?”

“We’re only concerned with one room, Kay. And that’s the room of origin. Without an origin, the rest of the fuel load doesn’t matter.”

“I see.”

“I know a flame was impinged on the ceiling in that bathroom, and I know how high that flame had to be and how many kilowatts of energy were needed for flashover. And a throw rug and maybe some towels and curtains couldn’t even come close to causing something like that.”

I knew her engineering equations were pristinely mathematical, and I did not doubt anything she was saying. But it did not matter. I was still left with the same problem. I had reason to believe that we were dealing with a homicide and that when the fire started, the victim’s body was inside the master bath, with its noncombustible marble floors, large mirrors, and steel. Indeed, she may have been in the tub.

“What about the open skylight?” I asked McGovern. “Does that fit with your theory?”

“It could. Because once again, the flames had to be high enough to break the glass, and then heat would have vented through the opening like a chimney. Every fire has its own personality, but certain behaviors are always the same because they conform to the laws of physics.”

“I understand.”