"Charred battery components indicated that the temperature inside the battery case exceeded 500 degrees," Hersman said.
But investigators still don't know what caused cell six to short-circuit in the first place.
They have ruled out two possibilities -- mechanical "impact" damage, like that caused by being dropped, or short-circuiting outside the battery.
But several other possibilities are being explored, including contamination or defect during manufacturing, flaws in the design or construction of the battery, and problems with battery charging. That final possibility -- battery charging -- leaves open the possibility that the problem could reside outside the battery itself.
When the FAA approved the use of the lithium-ion batteries on the Dreamliner, it imposed nine "special conditions" that were designed to prevent or mitigate problems.
In a joint statement on Thursday, Transportation Secretary Ray LaHood and FAA Administrator Michael Huerta reiterated that they are determined to fix the problem.
"Based on what information our experts find, the FAA will take any action necessary to further ensure safety. We must finish this work before reaching conclusions about what changes or improvements the FAA should make going forward," the statement read.
"The FAA is focused on the review and activities to understand the root cause. Once the review is complete, the FAA will make any analysis and conclusions public."
Boeing said it has hundreds of experts focused on solving the battery situation. "We are working this issue tirelessly in cooperation with our customers and the appropriate regulatory and investigative authorities."