Boston Marathon bombing suspect Dzhokhar Tsarnaev has told investigators his older brother Tamerlan was the driving force behind last week's attack and that no international terrorist groups were behind them, a U.S. government source said Monday. CNN says Tsarnaev, who has an injury to his throat, has communicated he and his brother acted alone and that Tamerlan, the older of the two, was the ringleader in the bombings. Preliminary interviews with Tsarnaev indicate the two brothers fit the classification of self-radicalized jihadists, the source said. Dzhokhar Tsarnaev, wounded and held in a Boston hospital, has said his brother -- who was killed early Friday -- wanted to defend Islam from attack, The 19-year-old, who could face the death penalty if convicted, was mostly silent and nodded affirmatively throughout the brief bedside hearing, indicating that he understood the charges laid against him, court transcripts published by The New York Times reveal. But when asked if he could afford a lawyer, the teen spoke for the first time, saying: "No". A doctor, identified in the transcript as Dr Odom, asked Tsarnaev how he was feeling. "Are you able to answer some questions?" the doctor asked the teen. Tsarnaev "nods affirmatively," according to the document the first of four times during the hearing. CNN