I believe that because the war was going on and papers were printing gossips about all his murders, he might pretty well tell them that Carlo died in this war with Barzini, bravely defending his family.
As to Fredo, nobody knew he had any hole in his head, because his body was not found. Michael didn't give orders to search it, and he remained peacefully at the bottom. It was such a stupid accident - to fall out of boat! But then, poor Fredo was always stupid...