The defence case has started in the trial of Boston Marathon bomber Dzhokhar Tsarnaev, as his lawyers tried to show his older brother masterminded the plan to detonate pressure-cooker bombs near the finish line.

Prosecutors rested their case against Mr Tsarnaev after jurors in his death penalty trial saw gruesome post-mortem photographs and heard a medical examiner describe the devastating injuries suffered by an eight-year-old boy killed in the 2013 terror attack.

One of the first witnesses called by the defence was a data analyst who said Mr Tsarnaev's mobile phone was being used in south-eastern Massachusetts - where he was attending college - while pressure cookers were being purchased north of Boston more than two months before the bombing.

The defence had made it clear from the first day of testimony on March 4 - when his lawyer admitted he participated in the bombings - their strategy was not to win an acquittal but to save Mr Tsarnaev from the death penalty by arguing that his brother Tamerlan was largely responsible for the bombings.

Prosecutors ended their case on an emotional note. At least three jurors cried and wiped their eyes with tissues as they looked at photos of eight-year-old Martin Richard, who went to watch the marathon with his parents and siblings, and was killed when the second of two bombs exploded near the finish line.

Martin's parents watched from the second row of the courtroom. Bill Richard kept his arm around the shoulder of his wife Denise throughout the testimony.

Two other people were killed and more than 260 were injured in the bombings. Prosecutors believe the brothers were seeking retaliation against the US for wars in Muslim countries.