Several media outlets reported earlier in the day that a suspect had been identified from surveillance video taken at a department store between the sites of the two bomb blasts, which killed three people and wounded more than 170.
A law enforcement official briefed on the investigation told The Associated Press that a suspect was in custody. The official, who was not authorized to divulge details of the investigation, said the suspect was expected in federal court.
But the FBI and the U.S. attorney's office in Boston said no arrests had been made.
"Contrary to widespread reporting, there have been no arrests made in connection with the Boston Marathon attack," the FBI said in a statement. "Over the past day and a half, there have been a number of press reports based on information from unofficial sources that has been inaccurate. Since these stories often have unintended consequences, we ask the media, particularly at this early stage of the investigation, to exercise caution and attempt to verify information through appropriate official channels before reporting."
The official who spoke to the AP did so on condition of anonymity and stood by the information even after it was disputed. A news briefing was planned later Wednesday.
A bomb threat forced the evacuation of the courthouse in midafternoon, the U.S. Marshals Service said, and security officials were sweeping the area.
Law enforcement agencies have pleaded for the public to come forward with photos, videos or any information that might help them solve the twin bombings. Police also gathered surveillance video from businesses around the finish line.
Multiple media outlets, citing unnamed sources, reported Wednesday that a video surveillance camera at a Lord & Taylor department store showed a suspect with a backpack near the marathon finish line. The store is between the sites of the two explosions.
An FBI agent has said investigators found pieces of black nylon from a bag or backpack and fragments of BBs and nails, possibly contained in a pressure cooker.
The bombs were made from ordinary kitchen pressure cookers packed with explosives, nails and ball bearings to inflict maximum carnage, investigators and others close to the case said. But the FBI said nobody had claimed responsibility.
Investigators in white jumpsuits had fanned out across the streets, rooftops and awnings around the blast site in search of clues on Wednesday. They combed through debris amid the toppled orange sports drink dispensers, trash cans and sleeves of plastic cups strewn across the street at the marathon's finish line.
President Barack Obama branded the attack an act of terrorism. Obama plans to attend an interfaith service Thursday in the victims' honor in Boston.
Scores of victims of the Boston bombing remained in hospitals, many with grievous injuries. Doctors who treated the wounded corroborated reports that the bombs were packed with shrapnel intended to cause mayhem. In addition to the 5-year-old child, a 9-year-old girl and 10-year-old boy were among 17 victims listed in critical condition.
The trauma surgery chief at Boston Medical Center says most of the injuries his hospital treated after the marathon bombings were to the legs.
"We have a lot of lower extremity injuries, so I think the damage was low to the ground and wasn't up," Dr. Peter Burke said. "The patients who do have head injuries were blown into things or were hit by fragments that went up."
Dozens of patients have been released from hospitals around the Boston area.
At Massachusetts General Hospital, all four amputations performed there were above the knee, with no hope of saving more of the legs, said Dr. George Velmahos, chief of trauma surgery.
"It wasn't a hard decision to make," he said Tuesday. "We just completed the ugly job that the bomb did."
The bombs exploded 10 or more seconds apart, tearing off victims' limbs and spattering streets with blood. The blasts near the finish line instantly turned the festive race into a hellish scene of confusion, horror and heroics.
The blasts killed 8-year-old Martin Richard, of Boston, and 29-year-old Krystle Campbell, of Medford. The Shenyang Evening News, a state-run Chinese newspaper, identified the third victim as Lu Lingzi. She was a graduate student at Boston University.
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