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.

Inside, a defense attorney for the 19-year-old ethnic Chechen enters a not guilty plea.

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.

Authorities say the bombs were assembled by Dzokhar and his 26-year old brother, Tamerlan, who was killed in a gun battle with police.

Dzhokhar was found a day later, badly wounded, hiding in a boat in a backyard.

According to court papers, he scrawled a note inside the boat that said the U.S. government is killing innocent Muslims.

The marathon attack injured about 264 people, with many losing legs.

Tsarnaev could face the death penalty if convicted.