The FBI could not be immediately reached for comment on the letter.
Advocacy group Fight for the Future has argued that by not telling Apple about the hack, the FBI is putting lives at risk. “If they really care about public safety, they must disclose the vulnerability they used to Apple to prevent criminals, hackers, and terrorists from exploiting the same security flaw and using it to do harm,” it said in a statement last week.
The Electronic Frontier Foundation has recommended a "very strong bias" in favor of an FBI disclosure to Apple, citing the Vulnerabilities Equities Process, a government policy for disclosing security vulnerabilities.
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