Dzhokhar Tsarnaev has told investigators his older brother Tamerlan was the mastermind of the Boston Marathon bombing and no international terrorist groups were involved, CNN  and NBC report a U.S. government source said Monday.

Preliminary interviews with Tsarnaev in his Boston hospital room indicate the two brothers fit the category of “self-radicalized” jihadists, the source said. Dzhokhar Tsarnaev has said his brother -- who was killed early Friday -- wanted to defend Islam from attack, according to the source.

The government source cautioned that the interviews were preliminary, and that Tsarnaev's account needs to be checked out and followed up on by investigators.

Newsday also reported that a federal law enforcement source with knowledge of the investigation said the surviving brother told investigators that he and his brother were motivated, at least in part, by religious beliefs.

The source said Tsarnaev told investigators the brothers learned how to make pressure cooker bombs online.