After recalling 2.5 million Galaxy Note 7 handsets and finally ceasing production of its flagship phablet earlier this week, Samsung still cannot get to the bottom of its exploding battery issue, according to a new report.

The South Korean company initially believed that defective batteries from a certain supplier were the problem, but after replacement units with new cells started catching fire, Samsung has been unable to pinpoint the cause of the issue.

It’s worrying news for Samsung smartphone fans.

The Note 7 was shaping up to be Samsung’s most successful smartphone. Pre-orders surpassed those for the Galaxy S7 and S7 edge combined, and the handset received rave reviews from pundits when it made its initial debut in late August.

[contextly_auto_sidebar] It looked like Samsung had perfected its smartphone formula, and with demand for the iPhone falling, it was exactly what the company needed to steal market share from Apple at the right time. But as we all know, that’s not how things panned out.

Instead, reports that the Note 7 had a tendency to explode started increasing, and Samsung was forced to recall every unit sold. Confident a “minor manufacturing flaw” at one battery supplier was to blame, Samsung quickly set about producing new units with new batteries.

Those arrived in late September, and they were quickly followed by Samsung’s worst fear: Customers found that even the new, “safe” Note 7 handsets were liable to overheating and blowing up. And no one knows why yet.

Samsung has been trying to replicate the issue for over a month, but engineers have been unable to get the Note 7 to explode, according to The New York Times. It seems clear that defective batteries weren’t the only problem, but Samsung has no idea what else could be to blame.

“It still has not disclosed what specifically caused the Note 7s to smoke and catch fire — or even whether it knows what the problem was,” reads the report.

Strategy Analytics has estimated that giving up on the Note 7 could cost Samsung as much as $10 billion, while $17 billion has already been wiped off its market value as shares in the company fell more than 8 percent earlier this week.

Fans and investors are now concerned that this problem could affect other Samsung products. If the company cannot pinpoint what is causing the explosions, it cannot completely eliminate the issue before it begins manufacturing new devices.

Samsung was quick to confirm a problem prior to its first recall, and it acted quickly to get defective devices off the market. However, it has been criticized for not responding quickly enough — and even trying to cover up — new reports that replacement devices are unfit for use.

It’s still unclear whether Samsung will now abandon the Note series altogether and launch a rebranded phablet lineup that doesn’t come with the same stigma attached. Either way, it needs to get to the bottom of this problem before it thinks about a Note successor.