Chinese intelligence agents acquired National Security Agency hacking tools and repurposed them in 2016 to attack American allies and private companies in Europe and Asia, a leading cybersecurity firm has discovered. The episode is the latest evidence that the United States has lost control of key parts of its cybersecurity arsenal.

Based on the timing of the attacks and clues in the computer code, researchers with the firm Symantec believe the Chinese did not steal the code but captured it from an N.S.A. attack on their own computers — like a gunslinger who grabs an enemy’s rifle and starts blasting away…

Some of the same N.S.A. hacking tools acquired by the Chinese were later dumped on the internet by a still-unidentified group that calls itself the Shadow Brokers and used by Russia and North Korea in devastating global attacks, although there appears to be no connection between China’s acquisition of the American cyberweapons and the Shadow Brokers’ later revelations.

But Symantec’s discovery provides the first evidence that Chinese state-sponsored hackers acquired some of the tools months before the Shadow Brokers first appeared on the internet in August 2016.