So far, the authorities have determined that people around Mr. Roof were aware that he held some racist beliefs.

Image Dylann Roof after he was arrested in connection with the shooting at Emanuel African Methodist Episcopal Church in Charleston, S.C., last month. Credit... Charleston County Sheriff's Office Detention Center

If the authorities find that any of Mr. Roof’s associates knew about his plans, they would most likely try to prosecute them. In the case of the 2013 Boston Marathon bombings, at least two men who were friends of the bombers were charged with helping them dispose of evidence from the bombing and from a subsequent shooting of a police officer. Three other men associated with the bombers were charged in connection with lying to the authorities or obstructing the investigation.

The authorities investigating the shooting that took place at Emanuel African Methodist Episcopal Church have been able to piece together the communications of Mr. Roof in part because they are in possession of his electronic devices, including his cellphone and computer. The devices were taken to F.B.I. laboratories in Quantico, Va., where they have been scrutinized by specially trained agents and analysts.

They showed that at least some of Mr. Roof’s communications with the hate groups were online.

The authorities also have looked over a manifesto and photographs posted online of him at Confederate heritage sites and slavery museums. The F.B.I. has concluded that they were probably posted by Mr. Roof. Although some of the photos appear to have been taken by someone else, bureau analysts have said that they were likely selfie photos or taken by Mr. Roof using a timer on the camera.

The State newspaper in Columbia, S.C., reported on Thursday that the investigation had widened to include others who may have helped Mr. Roof. It did not identify who those people were.