Federal authorities say the death of some of the walruses at a haul-out site in northwest Alaska were caused by humans, and the U.S. Fish and Wildlife Service believes it has identified those responsible.Walrus found dead and decapitated near Cape Lisburne. Photo: U.S. Fish and Wildlife Service.However, the U.S. attorney’s office, in a statement Friday afternoon, said charges have not been filed so far and the investigation continues. The statement said no further information on the investigation would be released at this time.Last month, federal authorities said they were investigating a report that about 25 walrus had been killed near Cape Lisburne and that some were missing tusks. Cape Lisburne is home to an Air Force radar station with few personnel.A message was left for an assistant U.S. attorney in Anchorage.