However, no arrest had been made, three separate US government and law enforcement sources told Reuters.
Police may make an appeal to the public for more information at a news conference scheduled for later on Wednesday, a US government source said.
Earlier, CNN reported that a suspect was in custody, citing law enforcement sources. But then the network cited three sources who said no one was under arrest after all.
The identification of a possible suspect marked the most significant publicly disclosed break since Monday's blast at the marathon finish line killed three people and injured 176 others.
Investigators were also searching through thousands of pieces of evidence from cell phone pictures to shrapnel shards pulled from victims' legs.
Based on shards of metal, fabric, wires and a battery recovered at the scene, the focus turned to whoever may have made bombs in pressure cooker pots and taken them in heavy black nylon bags to the finish line of the world-famous race watched by crowds of spectators.
A stretch of Boston's Boylston Street almost a mile long and blocks around it remained closed as investigators searched for clues in the worst attack on US soil since the hijacked plane strikes of September 11, 2001.
Cities across the United States were on edge after Monday's blasts in Boston. Adding to the nervousness was the announcement that mail containing a suspicious substance addressed to a lawmaker and to President Barack Obama. The FBI said, however, that agents had found no link the attack in Boston.
The blasts at the finish line of Monday's race injured 176 people and killed three: an 8-year old boy, Martin Richard, a 29-year-old woman, Krystle Campbell and a Boston University graduate student who was a Chinese citizen.
Boston University identified the student as Lu Lingzi.
No one has claimed responsibility for the attack.
"Whether it's homegrown, or foreign, we just don't know yet. And so I'm not going to contribute to any speculation on that," said US Secretary of State John Kerry, who until January was Massachusetts' senior senator. "It's just hard to believe that a Patriots' Day holiday, which is normally such time of festivities, turned into bloody mayhem."