An employee introduces a Samsung Electronics Galaxy Note 7 smartphone at a store in Seoul, South Korea. [Photo/Agencies]

SEOUL - Samsung Electronics said Monday that faulty battery caused its flagship Galaxy Note 7 to catch fire after discontinuing the fire-prone device more than three months ago.

In a special press conference in its headquarters in Seoul, Samsung said the mix of thin battery design and other manufacturing issues caused the Note 7s to explode or set on fire, which led to property damages and injuries.

The findings are based on two investigations by US-based firms UL and Exponent, which examined batteries and one supply-chain analysis by a German company TUV Rheinland.

The first group of the devices carried batteries with thin separators between the positive and negative layers that raise a possibility for internal short circuit, according to the UL's teardown examination.

Aggressively thin battery had been estimated by experts as one of the main reasons the flagship Samsung phone was overheated. Insufficient physical room can induce the positive and negative electrodes to touch and spark.

Sajeev Jesuda, one of the UL's executives, told reporters that higher energy density in batteries can exacerbate the severity of battery failure in "general" terms, falling short of the confirmation of one of reasons for explosions.

Deformation was found especially from the upper right corners of the batteries, weakening a protection capability from internal short circuit, the UL president said.

The second group of Note 7s was installed with batteries that have various manufacturing problems, including the missing insulation tape, irregular bumps and thin separators. Those factors led to internal short circuits.

According to the Exponent's analysis, no hardware and software issues have been discovered from the fire-prone devices.

The first group of phones showed deformation in upper corners of the defective batteries near a cathode tab. The second group had manufacturing issues, such as abnormally high bumps that can destroy an insulation tape and a separator.

Kevin White, a principal scientist at Exponent, told reporters that the first group suffered unintended damages to the cathode windings in the corner closest to the negative tab, saying it was caused by the pouch design.

The second group, he said, showed no deficiencies in the pouch, but welding defects in the positive electrode tab raised the short circuit possibility.

Meanwhile, no factor was found in the logistics and assembly processes that damage battery safety, according to the TUV Rheindland's investigations into factories in South Korea, China and Vietnam.

Holger Kunz, the German company's executive vice president, said its supply-chain analysis showed no specific defection of weakness, concern or obvious danger affecting battery safety integrity.