Facing charges in the worst mass-casualty attack on U.S. soil since 9/11, accused Boston Marathon bomber Dzhokhar Tsarnaev arrives at a federal courthouse in Boston.<br/> <br />Inside, a defense attorney for the 19-year-old ethnic Chechen enters a not guilty plea.<br/> <br />Tsarnaev has been charged with killing three people in April by setting off homemade pressure-cooker bombs in a crowd of thousands of race spectators, and later shooting dead a university police officer.<br/> <br />Authorities say the bombs were assembled by Dzokhar and his 26-year old brother, Tamerlan, who was killed in a gun battle with police.<br/> <br />Dzhokhar was found a day later, badly wounded, hiding in a boat in a backyard.<br/> <br />According to court papers, he scrawled a note inside the boat that said the U.S. government is killing innocent Muslims.<br/> <br />The marathon attack injured about 264 people, with many losing legs.<br/> <br />Tsarnaev could face the death penalty if convicted.
