WASHINGTON — American and French officials say there is still no definitive evidence to back up their presumption that the terrorists who massacred 129 people in Paris used new, difficult-to-crack encryption technologies to organize the plot.

But in interviews, Obama administration officials say the Islamic State has used a range of encryption technologies over the past year and a half, many of which defy cracking by the National Security Agency. Other encryption technologies, the officials hint, are less secure than terrorist and criminal groups may believe, and clearly they want to keep those adversaries guessing which ones the N.S.A. has pierced.

Some of the most powerful technologies are free, easily available encryption apps with names like Signal, Wickr and Telegram, which encode mobile messages from cellphones. Islamic State militants used Telegram two weeks ago to claim responsibility for the crash of the Russian jet in the Sinai Peninsula that killed 224 people, and used it again last week, in Arabic, English and French, to broadcast responsibility for the Paris carnage. It is not yet clear whether they also used Telegram’s secret-messaging service to encrypt their private conversations.