A law enforcement official briefed on the investigation told The Associated Press on Wednesday a suspect was in custody.
But the FBI and the U.S. attorney's office in Boston dispute that.
The official who spoke to The Associated Press did so on condition of anonymity and stood by the information even after it was disputed.
The official was not authorized to divulge details of the investigation. The official had said the suspect was expected in federal court in Boston.
Reporters and police have converged at the courthouse.