Samsung now blames both a design flaw and manufacturing problems for causing some of its "Galaxy Note Seven" phones to burst into flames.

A Samsung executive apologized Monday as he detailed the results of the company's investigation into what caused its Galaxy Note 7 smartphones to overheat and catch fire.

Samsung says since it began the recall last September, 700 researchers worked to replicate the problems using 200 thousand devices and 30 thousand batteries. They concluded the original issue was a design flaw.

The replacement phones had a problem with how the batteries were welded together. That led to an internal short circuit.

"They went through great pains to point out that in all of their testing, they didn't find any flaws with the actual design and build of the phone hardware or the software in it. They put all the blame strictly on the batteries and the manufacturers of those batteries,” says CNET Senior Writer Dan Ackerman.