Hackers who breached AT&T's systems may have handed criminals a roadmap to FBI informants, stealing call logs that could unravel investigations and jeopardize lives, according to alarming new details revealed Thursday.

A breach of AT&T that exposed “nearly all” of the company’s customers may have included records related to confidential FBI sources, potentially explaining the bureau’s new embrace of end-to-end encryption.

FBI leaders have warned that hackers who breached AT&T’s system last year likely stole months of agents’ call and text logs, prompting an urgent effort to safeguard confidential informants’ identities,