Sources say surveillance video taken by cameras at Lord & Taylor on Boylston Street helped identify a possible suspect placing a bomb.
On Wednesday afternoon, a law enforcement official told the Associated Press that the suspect was arrested and was expected in federal court shortly, but an FBI source confirmed to ABC News that no arrest has been made.
The FBI said it has received nearly 2,000 tips from around the world. Monday's attack killed three people and injured more than 170 people.
ABC News has obtained photos of what authorities are calling key pieces of evidence. They are images of a shredded backpack and what's left of a mangled Fagor brand pressure cooker pot that was packed with explosives, nails and other lethal shrapnel.
It appears one of the devices may have been put in the backpack and placed either inside or outside a trash can.
A Boston law enforcement source said identical pressure cooker bombs, each with a capacity to hold six liters of liquid, were set off.
"If there are pictures of the site when the bag isn't there, what they're going to try to do is, in their jargon, to 'negate the picture.' They're going to try to take pictures back and see when the bag was there and when it wasn't, so they can get the exact time of placement," said Richard Clark, a former national security official.
Experts said these clues suggest there may have only been one attacker, but they don't zero in on whether that attacker was from the U.S. or abroad.
Officials said the simplicity of the bomb makes it hard to trace it to any particular group. So far, no connection has been made to any terror group. Authorities don't have a sense of what the motive is, and no one is in custody.
Evidence recovered at the scene is being sent to the FBI's lab in Quantico, Va.
As of Tuesday morning, 76 people remain in six Boston hospitals.
Meantime, Boston Marathon organizers vowed to continue the race next year. They said the annual marathon is "a deeply held tradition and an integral part of the fabric and history of our community."
President Barack Obama and first lady Michelle Obama plan to be in Boston Thursday to attend a service for the victims.