(CNN) — Federal transportation investigators are still trying to figure out the cause of the electrical problems that have grounded Boeing's 787 Dreamliner fleet around the world.
The National Transportation Safety Board worked with manufacturers in Arizona in running a series of new tests on the airline's battery systems.
A battery-related fire on the tarmac in Boston triggered a review of the aircraft.
A second incident in Japan forced the shutdown.
Boeing says it's working around the clock to resolve the issue.