The focus of the FBI's investigation of the Boston Marathon bombings has shifted from identification and apprehension of suspects to piecing together details of the deadly plot.
Agents are trying to determine how long the planning took, how the plot was carried out and whether anyone else knew or was involved.
A federal law enforcement official, who asked to remain anonymous, said on Friday that the FBI is gathering evidence regarding "everything imaginable".
Two bombs exploded on April 15 near the marathon's finish line, killing three people and wounding more than 260.
Bombing suspect Dzhohkar Tsarnaev has been moved from a hospital to a federal prison medical centre. His brother died after a shoot-out.
Their mother says allegations against them are lies.