I have been reading a chapter in the book Forensic Science Reform written by John Lentini. Mr. Lentini quoted a portion of NFPA [National Fire Protection Association] 921:
24.2.1.2 Separate fires that are not caused by multiple deliberate ignitions can result from the following:
1. fire spread by conduction, convection, or radiation
2. fire spread by flying brands
3. fire spread by direct flame impingement
4. fire spread by falling flaming materials (i.e.m drop-down), such as curtains
5. fire spread through shafts, such as pipe chases or air conditioning ducts
6. fire spread within wall or floor cavities within “balloon construction’
7. overloaded electrical wiring
8. utility system failures
9. lightning
10. rupture and launching of aerosol containers
Mr. Lentini goes on to discuss the case of Han Tak Lee, who was convicted of setting a 1989 fire that was alleged to have nine separate points of origin. He was released in 2014 after it became clear that the testimony against him was fundamentally flawed. For one thing the room was fully involved, meaning that it had reached flashover.
I have read several news reports covering Mr. Mortimore's testimony. There is simply not enough information to verify whether or not he eliminated all other possibilities. Nor is there even a hint that anyone tested for an accelerant using gas chromatography/mass spectrometry.