Samsung Electronics Co. says 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 the company's investigation into one of its worst product fiascos comes three months after the flagship phone was discontinued.
The world's biggest smartphone maker recalled 2.5 million Note 7 phones in September after reports they were overheating and catching fire.
It blamed lithium batteries from a supplier.
New Note 7s with different batteries also caught fire.
So Samsung permanently dropped the premium phone in October.
It estimates the problems will cost it at least US$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.