US authorities denied media reports Wednesday that an individual had been arrested in connection to the Boston marathon bombings, but police said they do have images of a suspect.
Hundreds of investigators have been poring over surveillance video and still images taken around the finish line at the time of Monday's blasts, which killed three people and wounded 180.
CNN television reported that an arrest had been made. But the Federal Bureau of Investigation (FBI) and Boston police denied the account.
"Despite reports to the contrary there has not been an arrest in the marathon attack," the Boston police said on its Twitter account.
The two blasts went off in a 13-second period about 100 meters (yards) apart as stragglers from the marathon -- in which 23,000 competitors took part -- were crossing the finish line.
Investigators have told US media that some of images taken near the scene appear to show a man who may have planted the bombs.
The contradictory reports came ahead of a visit Thursday by President Barack Obama to honor the dead and injured.
With no claim of responsibility made for the attack, the Federal Bureau of Investigation (FBI) said it has launched a "worldwide" hunt for the attackers.
It 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 hotel's 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 was being kept aside for the 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, 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 will attend a special inter-faith service for the victims at Boston's Cathedral of the Holy Cross on Thursday morning.
Armed National Guard troops and police patrolled Boston's airport, commuter trains and buses, and authorities said the tightened security would last several days.
About 100 of the injured have left Boston hospitals but many remain in critical condition and the city has started emotional tributes to the dead, who include eight-year-old boy Martin Richard, restaurant manager Krystle Campbell and a Chinese graduate student at Boston University identified as Lu Lingzi.
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 in Boston.
Hundreds of students attended a memorial service for Lu at Boston University on Tuesday night. Thousands of tributes were posted online on Chinese websites.
A pair of running shoes, flowers and a key chain were left in front of the university memorial to Martin Luther King Jr., in honor of Lu.
Theology student Meghan Nelson, who placed the tributes, said: "All the theology school students are wearing running shoes this week. It symbolizes standing together for unity and running for peace. It's about solidarity."