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 not authorized to speak on the record about the investigation told The Associated Press on the condition of anonymity Friday the FBI is gathering evidence regarding “everything imaginable.”
Two bombs exploded 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 center. His brother died after a shootout. Their mother says allegations against them are lies.
Sullivan reported from Washington. Contributing to this report were Associated Press writers Rodrique Ngowi in Boston, Colleen Long in New York and Ted Bridis, Pete Yost and Julie Pace in Washington.