WASHINGTON, April 9 (Reuters) - Classified documents that appeared online, with details ranging from Ukraine's air defenses to Israel's Mossad spy agency, have U. officials said the investigation is in its early stages and those running it have not ruled out the possibility that pro-Russian elements were behind the leak, which is seen as one of the most serious security breaches since more than 700,000 documents, videos and diplomatic cables appeared on the WikiLeaks website in 2013. officials, speaking on condition of anonymity, said that while there was concern about the leak at the Pentagon and intelligence agencies, the documents showed a snapshot in time from more than a month ago, rather than more recent assessments