FBI agents zeroed in on how the Boston Marathon bombing was carried out - with kitchen pressure cookers packed with explosives, nails and other lethal shrapnel.
But they still did not know who did it and why.
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An intelligence bulletin released late yesterday includes a picture of a mangled pressure cooker and a torn black bag the Federal Bureau of Investigation says were part of a bomb.
The FBI and other law enforcement agencies urged members of the public to come forward with photos, videos or anything suspicious they might have seen or heard.
President Barack Obama branded the attack an act of terrorism but said officials do not know “whether it was planned and executed by a terrorist organisation, foreign or domestic, or was the act of a malevolent individual”.
Richard DesLauriers, FBI agent in charge in Boston, said that the “range of suspects and motives remains wide open”.
He vowed to “go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime”.
Scores of victims of the Boston bombing remained in hospitals, many with terrible injuries, a day after the twin explosions near the marathon’s finishing line killed three people, wounded more than 170 and reawakened fears of terrorism. A nine-year-old girl and 10-year-old boy were among 17 victims listed in critical condition.
Officials found that the bombs in Boston consisted of explosives put in ordinary, 1.6-gallon pressure cookers, one with shards of metal and ball bearings, the other with nails. Both were stuffed into black duffel bags and left on the ground.
Investigators said they have not yet discovered what was used to set off the explosives.
Pressure-cooker explosives have been used in international terrorism, and have been recommended for lone-wolf operatives by al Qaida’s branch in Yemen.
But information on how to make the bombs is readily found online, and officials said Americans should not rush to judgment in linking the attack to overseas terrorists.
Mr DesLauriers said there had been no claim of responsibility for the attack
He urged people to come forward with anything suspicious, such as hearing someone express an interest in explosives or a desire to attack the marathon, seeing someone carrying a dark heavy bag at the race, or hearing mysterious explosions recently.
The blasts killed eight-year-old Martin Richard of Boston, 29-year-old Krystle Campbell of Medford, Massachusetts, and a third victim, identified only as a graduate student at Boston University.
Mr Obama plans to visit Boston tomorrow to attend an interfaith service in honour of the victims.
In the wake of the attack, security was stepped up around the White House and across the country, but Homeland Security Secretary Janet Napolitano said that it was a precaution and there was no evidence the bombings were part of a wider plot.
Pressure-cooker explosives have been used in Afghanistan, India, Nepal and Pakistan, according to a July 2010 intelligence report by the FBI and the Homeland Security Department.
One of the three devices used in the May 2010 Times Square attempted bombing was a pressure cooker, the report said.
“Placed carefully, such devices provide little or no indication of an impending attack,” the report said.
The Pakistani Taliban, which claimed responsibility for the 2010 attempt in Times Square, has denied any part in the Boston Marathon attack.
The third person killed in the Boston Marathon bombings was a Chinese graduate student at Boston University, according to a state-run Chinese newspaper.
The Shenyang Evening News said on its official microblog account that the victim was Lu Lingzi, originally from China’s north-eastern city of Shenyang.
An editor at the newspaper said Ms Lu’s father confirmed his daughter’s death when reporters visited the family home.