FBI leaders have warned that they believe hackers who broke into AT&T Inc.’s system last year stole months of their agents’ call and text logs, setting off a race within the bureau to protect the identities of confidential informants, a document reviewed by Bloomberg News shows.FBI officials told agents across the country that details about their use on the telecom carrier’s network were believed to be among the billions of records stolen, according to the document and interviews with a current ...