The deadly Paris attacks have reignited debate on encrypted communications by terror cells and whether law enforcement and intelligence services are "going dark" in the face of new technologies.
The exact means of communication in Friday's strikes were not immediately clear, but media reports have said the Islamic State organisation has increasingly turned to encrypted communications and applications to avoid detection.
The latest carnage in France has revived concerns that law enforcement and intelligence lack the ability to tap into new communications technologies, even with appropriate legal authorisation.
CIA Director John Brennan, speaking at a Washington forum on Monday, warned that some technologies - without specifically mentioning encryption - "make it exceptionally difficult, both technically as well as legally, for intelligence and security services to have the insight they need to uncover it".
Brennan echoed concerns voiced by leaders of the FBI and National Security Agency that terrorists are using encryption to hide their tracks.
"I think what we're going to learn is that these guys are communicating via these encrypted apps, right, the commercial encryption, which is very difficult, if not impossible, for governments to break," former deputy CIA director Michael Morell told the CBS programme Face the Nation.
New York City Police Commissioner William Bratton echoed those concerns, saying his department is often frustrated by encryption - which has increased with new smartphones powered by Apple and Google software that provides only the users with keys to unlock data.
"We're encountering that all the time," Bratton told broadcaster MSNBC Monday (local time).
"We have a huge operation in New York City working closely with the Joint Terrorism Task Force and we encounter that frequently. We are monitoring [suspects] and they go dark. They are going onto an encrypted app, they are going onto sites that we cannot access. The technology has been purposely designed by our manufacturers so that even they cannot get into their own devices."
So far, the major US technology companies have spurned appeals from officials to enable access for key investigations and have stepped up encryption efforts following the 2013 leaks about vast surveillance capabilities of the US National Security Agency.
But in light of the bloodletting in France, the debate may change, observers say.
"Evidence that terrorists were, in fact, using strong end-to-end encryption to kill people could be game-changing in a debate that has heretofore been defined by anxieties about NSA," said Benjamin Wittes, a Brookings Institution fellow who edits the blog Lawfare.