An investigation into the cause of faulty batteries that led Samsung Electronics Co. to pull all of its Galaxy Note 7 smartphones last year found that some batteries were irregularly sized, causing overheating, while others had manufacturing problems, according to people familiar with the matter.

The conclusion, which will be unveiled by Samsung on Monday, helps to explain the technology giant’s product recall that damaged its brand and will end up costing the company at least $5 billion.

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