Testing at two Arizona facilities that supply electrical system components for the Boeing 787 has wrapped up with a declaration of "no significant findings."

Federal investigators visited Securaplane Technologies Inc. in Oro Valley and UTC Aerospace Systems in Phoenix as part of their inquiry into what caused a fire aboard an empty Japan Airlines 787 at a Boston airport earlier this month.

The fire affected the plane's auxiliary-power unit, or APU, one of two lithium-ion batteries used to power Boeing's high-profile, energy-efficient Dreamliner aircraft.

All 50 Dreamliners have been grounded since a smoking battery led to an emergency landing in western Japan later in the month and a U.S. Federal Aviation Administration review was launched.

Although U.S. National Transportation Safety Board Chairwoman Deborah Hersman said last week that investigators made "minor findings" during their Arizona tests, those findings have since been determined not to have been significant.

"Investigators take a lot of different readings and determine what is within the expected range," NTSB spokesman Peter Knudson said Monday. "No significant findings means there's no anomalies."

Investigators tested the auxiliary-power unit's start-power unit in Tucson and its controller in Phoenix. "Both units operated normally with no significant findings," according to the agency's Sunday news release.

The tests' completion does not mean the companies have been cleared of a role in the Dreamliner's battery problems, Knudson said. "We haven't yet reached any conclusions."

Investigators continue to disassemble and test the auxiliary-power unit at the NTSB lab in Washington, D.C.

They are testing battery cell elements with a scanning-electron microscope and energy-dispersive spectroscopy, and comparative tests on the plane's undamaged main battery have begun, the news release said.

Investigators have also paid a visit to another company producing components that interact closely with the 787's auxiliary-power unit - Japan's Kanto Aircraft Instrument Co., which manufactures the battery-monitoring unit used by Japan Airlines.

They examined the battery-monitoring unit circuit boards that were housed in the auxiliary-power unit's battery case, the news release said. Because the circuit boards were damaged, limited information could be gleaned from the tests. No significant discoveries were made.

Contact reporter Carli Brosseau at cbrosseau@azstarnet.com or 573-4197. On Twitter: @carlibrosseau.