Boston Marathon bombing: Mystery remains over motive

The motive for the Boston Marathon bombing remains unclear, despite charges being filed against surviving suspect Dzhokhar Tsarnaev.

Federal prosecutors charged him in hospital with using a weapon of mass destruction and malicious destruction of property resulting in death.
He could be sentenced to death if convicted on either count.
At the hearing, he managed to speak once despite a gunshot wound to his throat sustained during his capture.
Mr Tsarnaev, 19, said the word "no" when asked if he could afford a lawyer. Otherwise he nodded in response to Judge Marianne B Bowler's questions from his bed at Boston's Beth Israel Deaconess Medical Center.The next hearing in his case has been scheduled for the end of May.

Meanwhile security officials are expected to face questioning in the Senate Intelligence Committee about whether the FBI mishandled information on Mr Tsarnaev's 26-year-old brother Tamerlan.

He was interviewed in 2011 at the request of the Russian government based on concerns that he had become a follower of radical Islam. However, it appears no further action was taken.
Tamerlan, who is suspected of carrying out the attack along with his brother, was killed during a manhunt last Friday.
Boston observed a moment of silence for the victims at 14:50 local time (18:50 GMT) on Monday, exactly a week after the attack.
The twin bombs which exploded near the finishing line killed three people and injured more than 200.
Of those injured, 13 lost limbs. More than 50 people remained in hospital on Monday, three of them in critical condition.
Motive sought The 10-page criminal complaint filed against Mr Tsarnaev sets out the attack in detail.
It seeks to locate both suspects at the scene of the bombing and then pieces together the operation to intercept them three days later, as they drove a hijacked car near the city, hours after images of their faces were broadcast by the media.
No mention is made of their possible reasons for attacking the marathon.

Both men are known to be Muslims, with origins in the troubled republic of Chechnya in southern Russia. They had been living in the US for about a decade at the time of the attack.

AP's sources said they did not appear to have been linked to any Islamist militant groups.
Little has emerged to suggest the younger brother was a religious militant but the older man appears to have been drawn to radical Islam.
Last year, Tamerlan Tsarnaev spent six months in Dagestan, another mainly Muslim Russian republic bordering Chechnya. During the visit, he also reportedly spent two days in Chechnya itself.
An investigation by Radio Liberty has found evidence suggesting he lived on his own in Dagestan for two months, contrary to earlier reports that he was with his father the whole time.
'A little jolly girl' The funeral was held on Monday of 29-year-old restaurant worker Krystle Campbell, one of the three people killed in the bombing.
A memorial service was also held for Chinese graduate student Lu Lingzi, 23, at Boston University.
Her father, Lu Jun, thanked everyone for helping the family over the recent dark days.
"She was the family's Shirley Temple, if you will, the little elf and a little jolly girl, bringing everyone in the family ceaseless laughter,'' he said, speaking through an interpreter.
Ms Lu's roommate, Jing Li, addressed her words to her dead friend: "You need us to be strong and brave.
"We will keep running to finish the race for you and we will try to realise your unfinished dream."
A silence was observed across the state of Massachusetts, to be broken by the tolling of church bells.
In New York, stock exchange traders paused out of respect and commemorative events were held as far away as the Canadian capital Ottawa and French capital Paris.