US investigators on Wednesday said they found an image of a suspect who may have planted the homemade bombs that killed three people and maimed scores more at the Boston marathon.
The image could mark a breakthrough in the hunt for perpetrators, two days after the explosions that wounded 180 people. President Barack Obama and First Lady Michelle Obama will visit the city Thursday in a show of solidarity.
Boston remained on edge, as the federal courthouse and part of a hospital were evacuated, while conflicting reports of an arrest brought a stern rebuke from the Federal Bureau of Investigation (FBI).
A law enforcement official told AFP "there's an image of a person, of a potential suspect," and officials are working "to locate and identify that individual."
The Boston Globe said a department store surveillance camera and private photographer may have caught a suspect planting one of the two bombs set off at the finish line of Monday's marathon.
Separate media reports indicated there were pictures of the suspect running away while other people slumped to the ground in the blast.
With no claim of responsibility made for the attack, the FBI said it has launched a "worldwide" hunt.
Massachusetts Governor Deval Patrick told CNN the investigation "is going to be slow, it's going to be methodical," and asked the public to be patient.
"Not everything at every minute, with due respect to the curiosity of people, and the media, can be disclosed. Because some of that may compromise the investigation," he said.
The FBI meanwhile released photographs of the mangled metal remnants of a pressure cooker believed to have been used for one of the bombs, which sprayed nails, ball bearings and metal pellets into the huge crowds.
The lid of one pressure cooker was found on the roof of a nearby hotel, the owner told AFP. Shreds of black nylon bags believed to have been used to carry the bombs have also been found.
Doctors at hospitals where the critically injured were taken say ball bearings and nails taken from patients were being used in the investigation.
George Velmahos, Massachusetts General Hospital's chief of trauma surgery, said the metal was being handed over to police. He said 12 nails were taken from inside one patient.
Peter Burke, chief trauma surgeon at Boston Medical Center, also said the metal pieces were being set aside for police. He said some of the nails were about five centimeters (two inches) long.
Evidence is being collected for analysis at the FBI's main laboratory in Quantico, Virginia.
Similar, easy-to-make devices are used as roadside bombs in Afghanistan and Iraq, but have also been used by domestic extremists in the United States.
US authorities have thrown virtually every investigative agency into the hunt for Monday's bombers, with more than 1,000 officers working in Boston alone, said Rick DesLauriers, head of the FBI's Boston office.
Obama has condemned the attack as "an act of terror" and vowed that the attackers "will feel the full weight of justice."
The US leader and his wife Michelle will attend a special inter-faith service for the victims at Boston's Cathedral of the Holy Cross on Thursday morning.
About 100 of the injured have left Boston hospitals but many remain in critical condition and will require new operations.
Boston's federal courthouse and part of the Brigham and Women's Hospital, where many victims were taken, were evacuated on Wednesday.
Hundreds of people went to the courthouse after reports that a suspect had been arrested for the attacks.
But the reports were denied by Boston police and the FBI criticized what it called a number of "inaccurate" press reports since the attacks.
"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," it said in a statement.
Boston has held emotional tributes to the dead, who include eight-year-old boy Martin Richard, restaurant manager Krystle Campbell and Lu Lingzi, a Chinese graduate student at Boston University.
About 1,000 people attended a candlelight vigil in a park near the boy's home on Tuesday night and hundreds went to other events.
Hundreds of students attended a memorial service for Lu at Boston University and thousands of tributes were posted on Chinese websites.