The Ford F-150 Lightning fire that broke out at a Michigan factory in early February wasn’t caused by a battery defect, according to a report from Bloomberg. In a statement to the outlet, Ford’s South Korean battery supplier SK On calls the incident a “rare” occurrence and says it’s working on a fix.

Ford abruptly halted production of its flagship electric pickup on February 4th after one of the trucks caught fire in a holding lot affiliated with its Dearborn, Michigan, factory and spread to two other vehicles. Both Ford and SK have been investigating the issue for weeks and still haven’t resumed F-150 Lightning production. SK has once again started to build batteries in its Georgia factory, however.

“We believe this was a rare occurrence, not a fundamental issue with the technology of the battery cells or our overall manufacturing systems,” SK says in a statement to Bloomberg. “Working with Ford, SK On identified the root cause of the issue and implemented measures of improvement in our processes to address the issue.”

Details about what really went wrong here are still slim. At the time of the incident, Ford spokesperson Emma Bergg told The Verge that the company doesn’t “believe F-150 Lightnings already in customers’ hands are affected by this issue.” The battery fire has only directed more criticism toward the potential quality issues at Ford, which issued more recalls than any other carmaker in America last year.