Samsung is holding a press conference in three days to explain the defect in the Galaxy Note 7 that forced the product’s recall, but already it is being reported that wrongly sized batteries were what made some units combust.

As Samsung prepares to release its findings, a Wall Street Journal report citing “people familiar with the matter” claims the investigation found some devices were prone to overheating because they contained “irregularly sized” batteries that did not properly fit the phone. Beyond that core assessment, the Journal wrote that Samsung’s report also found that further manufacturing problems impacted so-called safe devices, which ultimately caused a full recall.

To recap: initially, when reports of customers’ devices exploding emerged, Samsung believed that the issue was related to phones with batteries produced by its Samsung SDI affiliate. Thus, it recalled the Samsung SDI devices and instructed its second manufacturer — Amperex Technology Ltd (ATL) — to increase its own production to keep up with the demand and cover replacement units.

However, the replacement units were deemed unsafe, too, after a number incidents and the Galaxy Note 7 was recalled in full and killed off for good. According to the Journal’s sources, ATL’s move to ramp up production led to unspecific “manufacturing issues” within its output, which forced the full discontinuation.

Samsung has since said that 96 percent of Galaxy Note 7 phones have been returned, thanks to extensive efforts to recall devices that include an update that bricks devices and airline warnings, but the entire saga is estimated to have cost it around $5 billion, not to mention dented its brand among consumers.

Post-Note 7, Samsung is introducing a more detailed testing regimen for its products to increase the level of inspection and raise quality assurances, according to the Journal’s report. That, the paper said, has been presented to government and regulatory officials in Washington who were said to have “responded positively.”

It remains to be seen how the Galaxy Note 7 recall will impact the sale of Samsung’s next high-end smartphone, but already there seems cause for optimism. Samsung’s latest financial report is out next week, with its current Q4 2016 quarter tipped to produce its best results for two years.