SEOUL, South Korea -- Samsung Electronics Co. said Monday that problems with the design and manufacturing of batteries in its Galaxy Note 7 smartphones caused them to overheat and burst into fire.
The announcement of results from the company's investigation into one of its worst product fiascos comes three months after the flagship phone was discontinued.
Seven-hundred researchers and engineers tested more than 200,000 devices and more than 30,000 batteries and replicated what happened with the Note 7 phones, the world's biggest smartphone maker said in a statement.
Samsung faulted design and manufacturing errors in two different battery types by two different manufacturers.
A highly technical explanation of the various problems boiled down to the relatively large batteries not fitting well into the phones, and not enough insulating material inside.
U.S. companies UL and Exponent examined the batteries and the German company TUV Rheinland analyzed the supply chain as part of the latest investigation, Samsung said.
Though it faulted batteries from its suppliers, the company said it was "taking responsibility for our failure to ultimately identify and verify the issues arising out of the battery design and manufacturing process."
The company has recalled 3.06 million Note 7 phones. The recalls began in September after reports the phones were overheating and catching fire. Samsung blamed a flaw in lithium batteries from one of its two suppliers.
New Note 7s with different batteries issued as replacements also caught fire. So Samsung permanently dropped the premium phone in October. It estimates the problems will cost it at least $5.3 billion through early 2017.
Samsung has taken heat for its handling of the recall and its hasty, apparently incomplete initial investigation into what went wrong.
The company has said it is planning the release of its next generation Galaxy phones. To do so, it needed to definitively resolve the mystery over the Note 7.
Samsung: Note 7 battery design, manufacturing caused fires