BRUSSELS — French counterterrorism investigators believe that the men suspected in last month’s Paris attacks used widely available encryption tools to communicate with each other, officials familiar with the investigation said, raising questions about whether the men used U.S.-made tools to hide the plot from authorities.

Investigators have previously said that messaging services WhatsApp and Telegram were found on some of the phones of the men suspected in the November attacks that claimed 130 victims. But they had not previously said that the services had been used by the men to communicate with each other in connection with the attacks. The two services are free, encrypted chat apps that can be downloaded onto smartphones. Both use encryption technology that makes it difficult for investigators to monitor conversations.

The findings of the investigation were confirmed by four officials, including one in France, who are familiar with the investigation. All spoke on the condition of anonymity because they were not authorized to speak publicly about the ongoing inquiry. A spokeswoman for the Paris prosecutor’s office, which is leading the investigation, declined to comment.

The investigators’ belief that WhatsApp and Telegram had been used in connection with the attacks was first reported by CNN.

The revelation is likely to add fuel to calls in Congress to force services such as WhatsApp, which is owned by Facebook, to add a back door that would enable investigators to monitor encrypted communications. Such demands have grown stronger in the wake of the Paris attacks and after other attacks in the United States in which the suspects are believed to have communicated securely with Islamic State plotters in Syria.

Already, security hawks in Congress, citing the likelihood that the Paris attackers used encrypted communications, have called for legislation. Sen. John McCain, R-Arizona, who chairs the Senate Armed Services Committee, has promised hearings on the issue.