At this point, no one is in custody in connection with the Monday afternoon attack that left three dead and sent 176 to area hospitals, Boston Police Commissioner Ed Davis said at a press conference. Of the injured, 17 remain in critical condition.
Late Monday, police searched a Boston area apartment of a Saudi Arabian student who was injured in the blast, law enforcement sources said.
On Tuesday, law enforcement sources briefed on the case said that the evidence was indicating that the Saudi student, who had been temporarily considered a "person of interest" in the investigation, would be cleared of suspicion and was unlikely to shed any light on the attack.
Numerous other theories and leads in the investigation are being looked at, the sources said, but at present there is no particularly strong lead or theory that is being pursued.
Richard Deslauriers, FBI special agent in charge of the investigation, declined to name any people being interviewed in the case.
White House officials and investigators said it was too early to say whether the Boston attacks were carried out by a foreign or home-grown group, or to identify a motive.
Dispelling earlier reports of as many as seven devices being found around Boston, Gene Marquez, assistant special agent in charge for the US Bureau of Alcohol, Tobacco and Firearms, said authorities had determined that the only bombs deployed in the attack were the two that detonated shortly before 3pm ET (1900 GMT) on Monday.
Any unexploded device might have provided a clearer picture of what materials were used and how the bomb was assembled, furnishing leads in the case.
Meanwhile, a stretch of Boylston Street near the race's finish line, where the blasts occurred, and the blocks around it were closed to traffic as police searched for evidence of the identity of who placed the bombs, which were packed with ball bearings to maximize casualties.
A banner that had marked the race's finish line still hung over the deserted street.
The White House said the bombings would be treated as "an act of terror," and President Barack Obama vowed that those responsible would "feel the full weight of justice."
It was the worst terror attack on US soil since security was tightened after the attacks of September 11, 2001.
Hospitals in the Boston area were planning surgeries for some of the victims, many of whom sustained lower leg injuries in the blasts, said Peter Fagenholz, a trauma surgeon at Massachusetts General Hospital.
"We're seeing a lot of shrapnel injuries" from small metal debris, Fagenholz told reporters outside the hospital. Doctors treated 29 people, of whom eight were in a critical condition.
An 8-year-old boy was among the dead, the Boston Globe reported, citing two law enforcement sources briefed on the investigation. A 2-year-old was being treated at Boston Children's Hospital for a head wound, the hospital said.
Major Cities on Alert
The blasts put police on alert in major cities across the United States, including Washington and New York City, the sites of the September 11 attacks.
The annual Boston Marathon, held since 1897, attracts an estimated half-million spectators and some 20,000 participants every year.
Officials in Britain and Spain said the London and Madrid marathons would go ahead on Sunday, but security plans for both races were under review.
"Since yesterday we are coordinating with municipal security and local government," Pedro Rumbao, Director of the Madrid marathon, told Spanish National Radio.
In Boston, runners who had travelled to the city for the race remained in shock on Tuesday morning.
Pat Monroe-DuPrey, of Winter Haven, Florida, ran with his wife, Laura, in a trip to mark their 10th anniversary after being married during the race.
He said he did not know what to make of the blast, which came as he was finishing the race in a state of exhaustion.
"You don't have a brain at 26 miles," Monroe-DuPrey said. "They got us off the course, and then I was panicking."