WASHINGTON (Reuters) - U.S. safety regulators are studying the certification process for the lithium ion battery on Boeing Co's 787 Dreamliner, after finding that short circuits in batteries can cascade to other cells, causing smoke and fire.

National Transportation Safety Board head Deborah Hersman told a press conference on Thursday that the agency will issue an "interim factual report" in 30 days, but said the decision to return the 787 to flight will be made by the Federal Aviation Administration.

The 50 Dreamliners in service have been grounded since January 16 while the NTSB, FAA and other aviation regulators around the world investigate the battery failures.

(Reporting by Jim Wolf, writing by Karey Wutkowski; Editing by Gerald E. McCormick)