SEATTLE, Wash. (Metro) -- Boeing's 787 Dreamliner fleet may be grounded for some time. The "Seattle Times" quotes National Transportation Safety Board chair Deborah Hersman as saying the agency hasn't reached a conclusion on the cause of a January 7 fire aboard a 787 in Boston. But she called the event "unprecedented" and said the NTSB is "very concerned." Hersman declined to say how long it would take to complete the investigation, saying there's a lot more work to be done. Copyright © 2013 Metro Networks Inc.