Boston Marathon attack motive still unclear

No mention has been made of any possible motive for the Boston Marathon bombing in the charges 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.

Boston observed a moment of silence for the victims on Monday, exactly one week after the attack.

The twin bombs which exploded near the finishing line killed three people and injured more than 200.

Jane Little reports from Washington.