A Samsung investigation into the Galaxy Note 7 smartphone batteries found the cause for the problem that led to the total recall of 2.5 million phones, say sources: irregularly sized batteries and others with manufacturing problems. Since some Galaxy Note 7 phones caught on fire, Samsung, which revealed the results of its investigation on Monday, saw damage to its brand and a loss of at least $5 billion. Led by Samsung, the investigation was conducted by three quality control and supply chain analysis firms. To avoid future mishaps, the company has developed a new QC process.

The Wall Street Journal reports that the batteries either came from “Samsung SDI Co., an affiliate, or Hong Kong-based Amperex Technology Ltd., which produces them at a factory in China.”

Earlier in its investigation, Samsung thought the battery problem only affected those made by its affiliate, and thus only recalled those devices. But as reports of problems continued, the company issued a second recall in October and “pull[ed] the plug entirely on the premium devices.” The ATL batteries were flawed due to “a manufacturing issue resulting from the quick ramp-up in production of replacement phones.”

According to WSJ, Samsung executives met with U.S. government officials “to discuss the findings,” and “the officials responded positively to Samsung’s presentation.” To prevent something similar happening in the future, Samsung now has an eight-step process “that includes more testing, inspections and manufacturing-quality assurances, among other measures.”

The outside firms that conducted the investigation were “U.S.-based companies UL LLC and Exponent Inc., which examined the batteries, while a German firm, TUV Rheinland, analyzed supply-chain issues.” Although 96 percent percent of the phones in the U.S. have been returned, the U.S. Federal Aviation Administration still bans the devices from airlines.

Elsewhere, WSJ gives Samsung a grade of “C” for now, with regard to its efforts to fix the battery issue, because “its explanation sometimes left us scratching our heads,” and its eight point battery check doesn’t give “a clear sense of whether these tests will raise the bar on safety, or simply catch Samsung up to other premium smartphone makers.”

The new tests will stress batteries with pressure and nail punctures, including “a machine that repeatedly discharges and charges batteries,” and “disassembling battery cells to inspect welding and the insulation tape conditions.” “Our concern is that … if Samsung doesn’t have a full answer to what led to its problem, it can’t effectively prevent it in the future.”

Future phone designs, says Samsung, will include “a bracket to provide additional battery protection when a phone is dropped.” The core of the problem was a lack of quality controls to identify the battery problems, something for which “Samsung takes full responsibility, and vows to change.”

Related:
Samsung Galaxy Note 7 Crisis Signals Problems at Korea Inc., The New York Times, 1/23/17