US investigators on Wednesday arrested a suspect in connection with the Boston marathon bombings that killed three people and wounded 180 others, CNN television reported.
The suspect was identified from surveillance video taken by a department store camera and another camera near the marathon finish line where two bombs sprayed metal fragments into crowds on Monday, CNN and other media reported.
At least one of the video cameras captured the suspect appearing to place one of the pressure cooker bombs, the reports said.
The two blasts went off in a 13-second period about 100 meters (yards) apart as stragglers among the 23,000 runners entered in the prestige event were completing the race.
The breakthrough came ahead of a visit to Boston on 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 those responsible.
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 hotel near the bombs, 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 investigation 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 warned that tight security would last several days.
About 100 of the injured have already left Boston hospitals but many remain in critical condition and the city has started emotional tributes to the dead, who include an eight-year-old boy Martin Richard, a Chinese graduate student at Boston University, Lu Lingzi, and a woman restaurant manager Krystle Campbell.
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 the woman 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 junior for 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."
London marathon organizers announced meanwhile that extra police would be deployed to monitor security following the Boston attacks.
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