WASHINGTON – The use of lithium ion batteries to power aircraft systems isn't necessarily unsafe despite a battery fire in one Boeing 787 Dreamliner and smoke in another, but manufacturers need to build in reliable safeguards, the nation's top aviation safety investigator said Wednesday.

National Transportation Safety Board Chairman Deborah Hersman said she doesn't want to "categorically" rule out the use of lithium ion batteries to power aircraft systems, even though it's clear that safeguards failed in the case of a Japan Airlines 787 that had a battery fire while parked at Boston's Logan International Airport last month.

"Obviously what we saw in the 787 battery fire in Boston shows us there were some risks that were not mitigated, that were not addressed," Hersman told reporters in an interview. The fire was "not what we would have expected to see in a brand new battery in a brand new airplane," she said.