The motive for the Boston Marathon bombings is still shrouded in mystery, despite charges having been filed against the surviving suspect of the twin blasts.
Federal prosecutors have already charged Dzhokhar Tsarnaev with using weapons of mass destruction and malicious destruction of property resulting in death.
The charges carry the death penalty or a prison sentence of up to life.
In addition to the federal charges, Dzhokhar is also likely to face state charges in connection with the shooting death of an Massachusetts Institute of Technology (MIT) police officer on April 18.
Dzhokhar and his brother Tamerlan, who was killed in a shootout with police on April 19, have been accused of carrying out the bombing attack near the Boston Marathon finish line on April 15, which killed three people and injured more than 200 others.
Investigators have said that the brothers have been motivated by their religious faith but have not produced any evidence to back up their claims.
The suspects' parents insist their sons are innocent and have accused the FBI of framing their children.
"I am sure that my kids were not involved in anything,” Zubeidat Tsarnaev has said.
The boys’ father, Anzor, also made a similar comment, saying that the two had been framed.
“I do not believe that my sons could have planned and organized the terrorist act, because they knew US national security services were keeping an eye on them,” the father said.
The grieving parents have cancelled a trip to the US, saying they would not be allowed to see their surviving son.