Boston Marathon bombing suspect Dzhokhar Tsarnaev has told investigators that no international terrorist groups were behind last week's attacks, a U.S. government source said Monday.

He also told investigators his older brother, Tamerlan, was the driving force behind the planning and execution of the attacks and wanted to defend Islam from attack, the source told CNN's Jake Tapper.

The government source cautions that this is just what the suspect is saying in these preliminary interviews, and that all of his claims need to be checked out and followed up by investigators.