It was already suspected, but now Samsung has given it the seal of approval. The nightmares around the Galaxy Note7 were due to the battery cells, and not fast charging or the wireless charging technology in the phones.
In a lengthy conference livestreamed earlier today, Samsung announced that it subjected thousands of Note7 batteries and devices to "repeated charge and discharge tests" as part of its manufacturing process. The firm discovered in a battery from the first recall that "incorrect positioning of the negative electrode tip", while a battery from the second batch of recalls was faulty due to "melted copper on [a] negative electrode", which caused a short circuit in that area of the battery.
Samsung hired three independent companies to investigate the Note7 failures: TUV Rheinland, Exponent, and UL. UL, which presented its findings during the livestream first, said it had found indications of internal short circuiting in the 10 damaged Note7 devices and 110 batteries they examined from the first of Samsung's two suppliers. The replacement batteries from the second supplier, meanwhile, also had missing insulation tape and multiple short circuits.
Exponent then said "the most likely root cause" of the battery failures from the first supplier was damage to the negative electrode windings, while the replacement batteries suffered from "a new and distinctly different defect" foreign to the original manufacturing.
TUV Rheinland, which investigated the logistics from Samsung's production lines in Vietnam and China as well as the assembly and transport, found there was "no specific detection of weakness, concern or obvious danger" from Samsung's processes in their South Korean and Vietnamese assembly lines, and that the 650 batteries they tested passed the requisite safety requirements after being transported via road.
If you want to read more in-depth analysis about the failure of Samsung's Note7 batteries, as well as a timeline of the saga until now, Cam from Gizmodo has got a great breakdown. Around 2500 Note7s are still in the wild, an intriguing figure given the extent Samsung has gone to cripple them (which includes cutting their maximum charge to 60%, disabling them from mobile networks and support for the Gear VR).