The key was obtained through access to the servers of the Russia-based criminal gang behind the July attack. Deploying it immediately could have helped the victims, including schools and hospitals, avoid what analysts estimate was millions of dollars in recovery costs.

But the FBI held on to the key, with the agreement of other agencies, in part because it was planning to carry out an operation to disrupt the hackers, a group known as REvil, and the bureau did not want to tip them off. Also, a government assessment found the harm was not as severe as initially feared...