The work to identify a bombing suspect from video and photo evidence continues, after Boston Marathon footage yielded a possible breakthrough Wednesday.
Investigators focused on a man seen dropping off a bag, and then walking away from the site of the second of two deadly explosions Monday that left three people dead and wounded more than 170.
The images found on surveillance footage from a department store near the finish line have not been made public.
U.S. President Barack Obama is attending an interfaith memorial service before noon Thursday in Boston. The president will be bringing a message of resolve and commonality, officials said.
On Wednesday, the FBI, federal prosecutors and Boston police all denied widespread media reports a suspect in the case had been arrested.
During a press conference, Massachusetts Governor Deval Patrick offered hope, saying the investigation is progressing.
Two blasts seconds apart near the finish line of Monday's marathon killed an eight-year-old boy, a restaurant catering manager and a Chinese graduate student at Boston University.