Smartphone companies tend to use third-party battery testing facilities, but Samsung’s exploding Note 7 batteries were actually tested in-house, reports The Wall Street Journal. In order to sell phones at major US carriers, batteries must be tested at one of 28 CTIA-certified labs in the US.

CTIA-certified labs ensure batteries meet the standards set by the Institute of Electrical and Electronics Engineers. In the case of Samsung, it operates its own CTIA-approved lab and may be the only manufacturer currently utilizing in-house testing.

During the lab tests, calling and charging scenarios are used to catch potential battery issues. Samsung’s lab didn’t detect any problems, but the Note 7 ended up catching fire or exploding in many cases, leading to a massive recall.

Samsung is still “working around the clock” to figure out what went wrong.