SEOUL – Smartphone giant Samsung has finally revealed that the reason behind explosion of its Note 7 phones was a defect in its battery.

Addressing a press conference on Monday, Dong-jin Koh, Samsung’s mobile business chief, acknowledged that two separate instances of battery malfunctions were to blame for some of the handsets catching fire. The Galaxy Note 7 handsets were packed with lithium-ion batteries.

“Our analysis showed the main cause for the incidents was deflections in the negative electrodes,” he said, adding incorrect positioning of the negative electrode tip also led to a higher likelihood of incidents.

The official also apologised to customers and suppliers for the defect that resulted in a major setback for the company aiming to tap into the Android market.

Some 2.5 million of the premium devices had been sold worldwide that needed to be recalled after the manufacturing defect, the firm had said. According to some financial experts the recall cost Samsung nearly $5 billion in lost revenue last year.

Major airlines had also advised commuters not to carry Samsung Galaxy Note7 while travelling due to the risks of battery explosions.

Samsung’s devices aren’t the only electronics to catch fire. Lithium-ion batteries, which currently power many mobile devices in addition to Samsung’s, have been known to catch fire. The U.S. Federal Aviation Administration has previously said they have recorded 171 incidents involving batteries aboard aircrafts from 1991 to January of this year.