The dust has settled around the Galaxy Note 7’s explosive end, and one question remains about Samsung’s recalled phone: Why did they actually explode? Thanks to a teardown of the device by engineering firm Instrumental, we may have the answer.

The small company believes it has found the errors which cause this flagship smartphone to catch fire and explode. And while we’ve yet to hear exactly what the cause was from Samsung themselves, this team build a pretty convincing case with the evidence they found.

So, what went wrong?

Simply put – in tech companies’ relentless chase to make phones more technically capable and thinner at the same time, Samsung didn’t leave enough room for the battery. In their own words: “the design can compress the battery even during normal operation.”

The lithium-polymer battery in the Note 7 looks like a flattened “jelly-roll,” which contains one positive layer made of lithium cobalt oxide and a negative one made of graphite – separated only by two electrolyte-soaked layers.

In making the battery in this device thinner, Samsung had to make these middle layers too thin and increased the chance of the positive and negative layers touching. If true, this would have been a key cause of the spark.

But the over-done space saving doesn’t end there. Instrument identified that when charging a mobile phone, the lithium layer of the battery will “mechanically swell” to roughly 10% bigger. Naturally, the extra space is needed in a battery’s case to compensate for this – but Samsung’s power unit completely filled the case it was in. This also increased the chance of explosion.

In fact, in a hypothetical world where the phones didn’t explode, Instrumental predict that the phone “slowly pushed apart by mechanical battery swell” over the course of a few years.

Before we call it terrible design…

The team didn’t just put this phone out with reckless disregard for the actual safety of the thing. Instrumental could tell this from that battery that sits within a CNC-machined pocket – with enough space built in around it to protect from other internal components.

This choice would have cost a lot of money for Samsung to put into practice, but it shows their engineers were “trying to balance the risk of a super-aggressive manufacturing process to maximize capacity, while attempting to protect it internally.”

What’s the judgement?

The big question – were Samsung knowingly putting dangerous phones in the hands of customers? Is there any chance they were deliberate in their actions?

In short, according to Instrumental, the answer seems to be “yes” and “no.”

To put out a product, you have to meticulously test every single element before hitting the market. Battery tests can take anywhere up to a year, and thousands of formulations are tested. It could be possible that “Samsung’s innovative battery manufacturing process was changing throughout development,” leading to a reduced amount of testing on the final version.

So, while it may have been a decision taken to benefit the design and capability of the phone, in Instrumental’s words: “They shipped a dangerous product.”

“Samsung took a deliberate step towards danger, and their existing test infrastructure and design validation process failed them.”