Accused Boston Marathon bomber Dzhokhar Tsarnaev pleaded not guilty to setting off a pair of homemade pressure-cooker bombs along with his brother at the race's crowded finish on 15 April.

The bombing attack killed three people and injured about 264. Tsarnaev and his older brother, 26-year-old Tamerlan, are also accused of killing a fourth person, a campus police officer, three days later when they prepared to flee Boston.

Tamerlan Tsarnaev died in an overnight gunbattle with police after killing the officer.