Officials: Surviving Suspect Says He And Brother Acted Alone

A clearer motive is beginning to emerge eight days after a heinous attack on the Boston Marathon.

The surviving suspect has been cooperating with authorities, answering questions in writing due to a throat injury that has rendered him unable to speak.

U.S. officials say Dzhokar Tsarnaev's answers indicate his increasingly radicalized brother was the mastermind behind the attack, and outside of involving his brother, acted alone. There does not appear to be any links to terrorist organizations.

Sources tell CNN that preliminary interviews with Tsarnaev indicate that his brother acted on the idea that Islam is under attack and jihadists need to fight back. The brothers appear to fit the classification of self-radicalized jihadists, though the source cautions against making any premature conclusions, as Tsarnaev's account still needs to be checked out and followed up by investigators.

Tsarnaev was formally charged Monday. The 19-year-old has been charged with one count of conspiring to use a weapon of mass destruction resulting in death, and one count of malicious destruction of property by means of an explosive device resulting in death.

The White House said that under U.S. law, American citizens cannot be tried in military commissions, meaning Tsarnaev will be prosecuted in the federal court system rather than as an enemy combatant in a military tribunal.

Tsarnaev remains under heavy guard at Boston's Beth Israel Deaconess Medical Center, where 11 victims of the bombing are still hospitalized. He is in serious, but stable condition, suffering multiple gunshot wounds.