While some Americans’ phone calls and emails may have been compromised by the Chinese, officials emphasized that encrypted applications, like WhatsApp and Signal, were not penetrated. In addition, messages sent within Apple’s own network were also safe.

And the discovery of the specific targeting of senior national security officials, and some political leaders — including President-elect Donald J. Trump and Vice President-elect JD Vance — led the F.B.I. and other officials to conclude that the Salt Typhoon hackers were so deep in the system that they could actually listen in to some conversations and read some unencrypted text messages.