Note: Updated on Jan. 23 to incorporate information from Samsung.

An investigation by Samsung Electronics Ltd. has determined a series of “abnormal” batteries were officially behind last year’s Galaxy Note 7 explosions.

“We learned from the Galaxy Note 7 issues and have made changes as a result,” the South Korean tech giant wrote in a public notice that appeared on its U.S. website on Jan. 22. “From reassessing every step of our smartphone manufacturing process to redesigning our quality assurance program, we are committed to implementing every learning to ensure quality and safety going forward.”

In a Jan. 23 press conference, accompanied by a same-day press release, the company released a detailed series of diagrams illustrating the precise factors that made both series of Note 7 batteries – “Battery A,” issued before the device’s initial recall, and “Battery B,” released after – abnormal.

According to the Wall Street Journal, which leaked several details from the report on Jan. 20, citing “people familiar with the matter,” the issues were caused both by an irregularly sized batteries and “manufacturing issues,” depending on the manufacturer.

“The issue with the batteries from Samsung SDI was an irregularly sized battery that didn’t fit properly in the phone, according to the people, who said that the incongruence caused the overheating,” WSJ reporters Timothy W. Martin and John D. McKinnon wrote. “In the Galaxy Note 7 phones carrying batteries made by ATL, the flaw centers on a manufacturing issue resulting from the quick ramp-up in production of replacement phones, these people said. It wasn’t clear what the manufacturing issues were.”

To reach its conclusions, Samsung enlisted 700 engineers to conduct months of testing on more than 200,000 devices and 30,000 batteries, and enlisted three industry organizations – UL, Exponent and TÜV Rheinland – to conduct independent analyses of their own.

The Galaxy Note 7, which was well-reviewed when it hit the market in August, carried batteries from one of two suppliers: Samsung affiliate Samsung SDI Co. or Hong Kong-based Amperex Technology Ltd., which has production facilities in China. Though Samsung officials initially believed that only batteries manufactured by Samsung SDI were overheating, and recalled those devices while ramping up production of Amperex battery-powered smartphones to replace them, reports of fires continued and Samsung issued an official recall of all devices in September.

According to the Wall Street Journal, the combined expense of the Note 7 recall and its damage to Samsung’s brand could end up costing the company at least $5 billion USD.

Amperex and Samsung SDI declined to comment on the Wall Street Journal story.

Samsung had also been in the news recently thanks to a high-profile investigation of vice chair and de facto leader Lee Jae-yong, who was accused by South Korean authorities for allegedly conspiring with impeached South Korean president Park Geun-hye to further his control of the company. However, the authorities’ request for an arrest warrant was denied on Jan. 19.

Check out the infographic Samsung released indicating precisely why the Note 7’s batteries overheated below (click for a larger version).

Infographic-Galaxy-Note7-What-we-discovered

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