National Transportation Safety Board Chairman Deborah Hersman points to a reporter during a news conference at the NTSB in Washington, Thursday, Jan. 24, 2013. The Boeing 787 Dreamliner battery that caught fire earlier this month in Boston shows evidence of short-circuiting and a chemical reaction known as "thermal runaway," in which an increase in temperature causes progressively hotter temperatures, federal accident investigators said. -It's not clear to investigators which came first, the short-circuiting or the thermal runaway, Hersman said. Nor is it clear yet what caused either of them, she said. (AP Photo/Manuel Balce Ceneta)

Associated Press
National Transportation Safety Board Chairman Deborah Hersman points to a reporter during a news conference at the NTSB in Washington, Thursday, Jan. 24, 2013. The Boeing 787 Dreamliner battery that caught fire earlier this month in Boston shows evidence of short-circuiting and a chemical reaction known as "thermal runaway," in which an increase in temperature causes progressively hotter temperatures, federal accident investigators said. -It's not clear to investigators which came first, the short-circuiting or the thermal runaway, Hersman said. Nor is it clear yet what caused either of them, she said. (AP Photo/Manuel Balce Ceneta)
National Transportation Safety Board Chairman Deborah Hersman points to a reporter during a news conference at the NTSB in Washington, Thursday, Jan. 24, 2013. The Boeing 787 Dreamliner battery that caught fire earlier this month in Boston shows evidence of short-circuiting and a chemical reaction known as "thermal runaway," in which an increase in temperature causes progressively hotter temperatures, federal accident investigators said. -It's not clear to investigators which came first, the short-circuiting or the thermal runaway, Hersman said. Nor is it clear yet what caused either of them, she said. (AP Photo/Manuel Balce Ceneta)
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