In early January, a Boeing 787 Dreamliner flying for Japan Airlines caught fire at Boston’s Logan International Airport. Eight days later, another 787 flying for All Nippon Airlines had to make an emergency lading when batteries again showed signs of failure. These battery scares led to the grounding of Boeing’s 50-plane Dreamliner fleet—a fleet that was supposed to represent the future of air travel, using 20 percent less fuel than similarly heavy planes.

Now, investigators at the National Transportation Safety Board have determined the cause of the battery fire in the Japan Airlines 787, saying that the fire was caused by a short circuit in one of the battery’s cells leading to a thermal runaway which led to the other cells catching fire. The NTSB said that temperatures inside the battery exceeded 260 degrees Celsius (500 degrees Fahrenheit).

As Ars reported in January, the Dreamliner’s lithium batteries had the potential to reach that state of thermal runaway—a situation that can turn a battery into a firebomb—at 140 degrees Celsius.

While the safety board was still trying to find the exact cause of the short circuit, the board criticized the process that the FAA and Boeing had used to approve the safety of the battery in the Dreamliner planes. According to the blog Frequent Business Traveler, NTSB chairman Deborah Hersman said that “assumptions used to certify the batteries must be reconsidered” and until then, the Dreamliner planes would not to return to passenger service. Hersman also commented that Boeing’s tests of the battery had failed to find that a short circuit in one cell could become a problem in the other battery cells, which is exactly what had happened in January.

Beyond the obvious problem with firebomb-like situations on airplanes, the Dreamliner model also depends very heavily on electrical systems (“the plane's avionics are wired together with a derivative of 100 megabit Ethernet to reduce the amount of wiring and corresponding weight,” reporter Sean Gallagher wrote). Frequent Business Traveler said that the Dreamliner uses electrical systems in "de-icing the wings, pressurizing the cabin, and operating hydraulic pumps," among other tasks.

The NTSB is continuing its investigation into the cause of the shorted circuit.