Federal Bureau of Investigation (FBI) agents have said the Boston Marathon bombing was carried out with kitchen pressure cookers packed with explosives, nails and other lethal shrapnel, but said they still didn't know who did it and why.
An intelligence bulletin issued to law enforcement and released late on Tuesday included a picture of a mangled pressure cooker and a torn black bag the FBI said were part of a bomb.
Earlier, law enforcement officials said they had recovered forensic evidence that suggested the two explosive devices which ripped through participants of the marathon on Monday may have been inside heavy black nylon bags.
The FBI and other prominent law enforcement agencies repeatedly pleaded for members of the public to come forward with photos, videos or anything suspicious they might have seen or heard.
Barack Obama, the US president, branded the attack an act of terrorism but said officials didn't know "whether it was planned and executed by a terrorist organization, foreign or domestic, or was the act of a malevolent individual".
Speaking at a joint law enforcement news conference on Tuesday, Richard DesLauriers, the FBI special agent in charge of the case, said that investigators had received "voluminous tips" and were interviewing witnesses and were analysing the crime scene.
DesLauriers pledged "we will go to the ends of the Earth" to find whoever carried out the deadly attack on one of the city's most famous civic holidays, Patriots Day.
Authorities served a warrant on a suburban Boston home and appealed for any images or audio of the blasts that killed three people and injured 176.
Following the bombings, major cities across the US have been placed on high alert.
Amid heightened security across the country, Harry Reid, the Senate majority leader, said a letter containing ricin or another poision had been sent to the office of Senator Roger Wicker of Mississippi.
I think it's a good showing. It's nice to see people in this community right here
Hundreds gathered Tuesday around Parkman Bandstand on Boston Common for a vigil that aimed to bring peace to a community shaken by the bombings that shook the city's marathon on Monday.
In the quiet crowd visitors fought back tears as then embraced friends and loved ones in the gathering to honor the victims of the Boston Marathon explosion that has taken three lives and wounded 176 others.
"I think it's a good showing. It's nice to see people in this community right here," said Kate Bergstrom, a Boston resident.
Friends were seen crying on a caring shoulder as others held each other as they sang songs of hope and love to honour and remember the victims of Monday's horrific events.
Dozens lit candles and laid flowers on the ground during the quiet and somber event.
The explosions at the marathon took place about 10 seconds and about 90 metres apart, knocking spectators and at least one runner off their feet.
[X-ray of one victim's leg that had] what appears to be small, uniform, round objects throughout it - similar in the appearance to BBs
Al Jazeera's Alan Fisher, reporting from Boston, said such a bomb was set to explode by using a mobile phone and had been used in Iraq and Afghanistan.
"The police were saying that there were traces of pressure cooker found at the site," he said.
"The suggestion is that the pressure cooker was put in a backpack and placed on the ground, and that is why we've got so many lower body injuries."
Our correspondent said that would explain why there was a small blast which caused a great deal of damage to the people nearby, as the shrapnel spread over a wide area.
Deval Patrick, governor of Massachusetts, said that no unexploded bombs were found at the marathon. He said the only explosives were the ones that went off on Monday.
On MOnday, the Massachusetts state police confirmed that a search warrant related to the investigation into the explosions was served in Revere, but gave no further details.
Some investigators were seen leaving the Revere house early on Tuesday carrying brown paper bags, plastic trash bags and a duffel bag.
Dr Stephen Epstein of the emergency medicine department at Boston's Beth Israel Deaconess Medical Centre, said he saw an X-ray of one victim's leg that had "what appears to be small, uniform, round objects throughout it - similar in the appearance to BBs", referring to ball bearings.
Police said three people were killed. Eight-year-old Martin Richard was among the dead. The boy's mother, Denise, and six-year-old sister, Jane, were badly injured in the blasts.
Boston Police Commissioner Ed Davis said 17 people were in a critical condition.
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