MINNEAPOLIS (AP) – Federal safety investigators intensified their scrutiny of a Monday fire aboard a Boeing 787 as concerned investors sold shares in the aircraft maker for a second day.

Boeing on Tuesday confirmed that the fire aboard a Japan Airlines plane appeared to have started in a battery pack for the plane’s auxiliary power unit.

The National Transportation Safety Board described the fire damage as “severe.” and said it is sending two more investigators to examine the Japan Airlines plane. It also formed investigative groups to look at the plane’s electrical systems as well as the fire response.