A U.S. investigation into a leak of hacking tools used by the National Security Agency is focusing on a theory that one of its operatives carelessly left them available on a remote computer and Russian hackers found them, four people with direct knowledge of the probe said.
The tools, which enable hackers to exploit software flaws in computer and communications systems from vendors such as Cisco Systems and Fortinet Inc., were dumped onto public websites last month by a group calling itself Shadow Brokers.
The public release of the tools coincided with U.S. officials saying they had concluded that Russia or its proxies were responsible for hacking political party organizations in the run-up to the Nov. 8 presidential election. On Thursday, lawmakers accused Russia of being responsible.