A group engineers from the manufacturing technology company Instrumental tore down a Galaxy Note 7 to try and figure out what may have caused the devices to overheat and explode, causing Samsung to recall and eventually cancel all Galaxy Note 7 devices. This is what they found.

They discovered the battery was so tightly packed inside the Galaxy Note 7’s body that any pressure from battery expansion, or stress on the body itself, may squeeze together layers inside the battery that are never supposed to touch — with explosive results.

Batteries swell up under normal use, and we place stress on a phone’s body by putting it our pocket and sitting down, or if it’s dropped. Tolerances for battery expansion are built into a smartphone during design, and Instrumental notes Samsung used “a super-aggressive manufacturing process to maximize capacity.” In other words, the Galaxy Note 7 was designed to be as thin and sleek as possible, while containing the maximum battery capacity for long use, thereby better competing against rival devices such as the iPhone 7 Plus and improving on previous Note models.

The report speculates that any pressure placed on the battery in its confined space may have squeezed together positive and negative layers inside the cell itself, which were thinner than usual in the Note 7’s battery already, causing them to touch, heat up, and eventually in some cases, catch fire. Delving deeper into the design, the engineers say the space above a battery inside a device needs a “ceiling” that equates to approximately 10 percent of the overall thickness. The Galaxy Note 7 should have had a 0.5mm ceiling; it had none.