The newspaper cited people who were familiar with the matter and said that Samsung had hired three companies that were experienced in quality control and analysis of supply chain mechanisms to aid in the investigations.
The company has announced that it will make an official announcement of the reason for the failure of the device on Monday, 23 June.
The Note7 was released in August last year. A month later, reports of exploding devices and resultant fires started to pour in.
Batteries for the Note7 were supplied by Samsung SDI, an affiliate, and Amperex Technology which is based in Hong Kong. Initially it was thought that only devices with the Samsung SDI batteries were prone to explode. This proved to be wrong.
The WSJ report said Samsung's official report on Monday would say that batteries from Samsung SDI were of irregular size and did not fit properly.
In the case of phones with batteries made by Amperex, a manufacturing issue, caused by an increase in the pace of production necessitated by the need for replacement phones to replace those which had exploded, was held to be the cause of the fires. The exact issue was not identified.
Samsung has created an eight-step process which encompasses more testing, inspections and manufacturing quality assurance steps to address any concerns that regulators have about problems in the future.
Apart from Samsung's own staff, personnel from the American firms UL and Exponent helped to find the battery issues, while Germany's TUV Rheinland looked at supply chain problems.
The Note7 fiasco is estimated to cost Samsung about US$5 billion but the company is due to announce its biggest quarterly profit on Tuesday.