The attackers are believed to have communicated using encryption technology, according to European officials who had been briefed on the investigation but were not authorized to speak publicly. It was not clear whether the encryption was part of widely used communications tools, like WhatsApp, which the authorities have a hard time monitoring, or something more elaborate. Intelligence officials have been pressing for more leeway to counter the growing use of encryption.

The disciplined way some of the attackers handled themselves during the assault and evidence of some military-style training — for example, having one attacker continue shooting while another reloaded his weapon — also suggested that the plot involved considerable planning and input from an organized group, a French official said.

Following the attacks, intelligence and law-enforcement agencies reviewed intercepted communications and concluded that the attackers had been in touch with members of the Islamic State in Syria, American and French officials said.