The Boston Marathon bombs were built inside pressure cookers and packed with shards of metal, nails and ball bearings, it emerged last night.
The aim was to cause maximum carnage as red-hot shrapnel tore through spectators and runners.
Investigators believe the explosives – which killed three and injured more than 170 – were in six-litre pressure cookers with gunpowder and set off by an electronic timer.
Bloodied: Injured runners and spectators lie on the pavement after two bombs exploded near the finish line of the Boston Marathon on Monday
Investigators continue to comb the scene after the horrific attack in Boston
It is believed the six-litre pressure cooker bombs were set off by an electronic timer
The devices are used by terrorists in Afghanistan, are a preferred weapon of al-Qaeda and listed as the ‘most effective’ weapon of jihad.
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Details of the crude but effective devices emerged as President Barack Obama told the United States to be on the alert for anything suspicious and admitted it was still not known who was responsible.
He said the Patriots’ Day blasts were ‘a heinous and cowardly act’ of terror used to target innocent people and promised those responsible would feel the ‘full weight of justice’.
But Mr Obama also said it was unclear if the attacks were carried out by an international or domestic organisation, or perhaps by a ‘malevolent individual.’
One of the bombs was placed in a black back-pack before being put on the ground among spectators, the other was left in a dustbin. One may have been triggered by a mobile telephone.
The pressure cooker devices have been used in Afghanistan, India, Nepal and Pakistan.
At least one was employed in the attempted May 2010 bombing of Times Square in New York by Faisal Shahzad, who admitted he had undergone bomb-making training at a militant Islamist faction camp in Pakistan.
However, the Pakistani Taliban, which claimed responsibility for the 2010 attempt in Times Square, has denied any role in the Boston attack.
A man kneels praying in front of a makeshift memorial on Boylston Street, Boston
Marathon runners embrace at the gates where three people died and more than 170 were injured in the bombing
Three people, including an eight-year-old boy, were killed and 176 injured, 17 critically, in the blasts near the finishing line of the showpiece event watched on TV by millions across the United States on Monday.
Yesterday Richard DesLauriers, of the FBI, said: ‘We will go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime.’
Officers, who checked the course before the marathon started, now think the bomber had a ‘three-hour window’ to plant the devices before the blasts and are looking into a number of suspect groups and theories.
The include the possibility of a fanatical right-winger or ‘Lone Wolf’. The crude, basic nature of the bombs are said to point towards this and the attack was launched on April 15, tax day in the United States, when the whole nation is expected to file tax returns.
Experts speculated right-wingers could have seen the marathon as a liberal, multicultural bonanza worthy of targeting.
Significantly, there has been a large rise in the number of anti-government or so-called ‘Patriot’ groups during Mr Obama’s presidency.
Monday was Patriots’ Day in the US, a public holiday commemorating the anniversary of the Battles of Lexington and Concord in 1775, the first battles of the War of Independence, which would be cause for celebration for such groups.
The obvious alternative theory is a foreign terrorist group. Al Qaeda, the Taliban and terrorists linked to the groups have been desperate to carry out a bombing on the US mainland, especially since Special Forces shot Osama Bin Laden dead in his Pakistan hideout.
The devices have been used in Afghanistan, India, Nepal and Pakistan but no group has yet claimed responsibility for the attack
The FBI has applied for a warrant to extract the identity of every mobile telephone in the area where the bombings took place
But their efforts have been repeatedly foiled and Monday’s attacks are not thought to be the work of suicide bombers.
Al Qaeda and their associates are usually swift to claim responsibility for their attacks but last night no group had come forward.
Yesterday the full extent of the horrific injuries suffered by the victims became clear as Dr George Velmahos, of Massachusetts General Hospital, said his doctors removed ‘a variety of sharp objects,’ including pellets and nails, from the wounds of victims – one had 40 pellets in a leg.
His colleague, Dr Stephen Epstein, disclosed 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 ball bearings’.
The FBI has applied for a warrant to extract the identity of every mobile telephone in the area where the bombings took place and has requested that all spectators hand over any phone or camera footage to help try to identify any suspects.
Last night police said that no one was under arrest but refused to provide any insight into a raid on a residential building in Revere, eight miles north-east of Boston, late on Monday – said to be the home of a Saudi Arabian man.
President Obama told the United States to be alert for anything suspicious and admitted it is still not known who was responsible for the Boston bombing
Several large bags were taken from a fifth-floor apartment early yesterday.
Police, who confirmed explosives experts had accompanied them in the search, which was done ‘under warrant’, were earlier said to have questioned two men from Saudi Arabia.
The leader of the 9/11 attacks and several members of the cells that hijacked four planes were from Saudi Arabia, but officials ‘cautioned’ against putting ‘great significance’ on the raids ‘at this stage’.
A 20 year-old student linked to the address is said to be under police guard in hospital, where he is being treated for burns and shrapnel wounds to his legs, after he was tackled to the ground by a civilian who believed he was acting suspiciously.
At Boston’s Brigham and Women’s Hospital, investigators seized the man’s clothes to examine whether they held any evidence that he was linked to the attack.
Law sources told the New York Post that after the man was grabbed by police, he smelled of gunpowder and said, ‘I thought there would be a second bomb’, before asking, ‘Did anyone die?’
Patriot's Day, a black day for America
By CHRIS GREENWOOD
The Boston Marathon attacks will go down in history as yet another atrocity associated with Patriots’ Day.
Investigators fear the timing of the bomb blasts on a day full of significance for many Americans is no coincidence.
The celebration of freedom marks the anniversary of the first battles of the American Revolution in 1775 – the ‘shot heard around the world’.
But its meaning has been warped by a growing number of white supremacists and other far-Right groups who have turned it into a rallying point. The Boston bombings join a litany of other terrible events connected to the day, including:
The Waco siege which left 76 dead at self-declared messiah David Koresh’s compound on Patriots’ Day in 1993.
White supremacists blew up a federal building in Oklahoma City on April 19, 1995, killing 168. Bomber Timothy McVeigh, who was executed in 2001, planned the blast to coincide with the second anniversary of Waco.
The 1999 Columbine school massacre, which left 15 dead and 21 injured, took place on the day after Patriots’ Day. Gunmen Eric Harris and Dylan Klebold murdered 12 students and one teacher before killing themselves.
The Virginia Tech massacre, the deadliest school shooting in US history, took place on April 16, 2007. Seung-Hui Cho, a senior student, shot and killed 32 people and wounded 17 others in two separate attacks before committing suicide.
Patriots’ Day is April 19, but in Massachusetts and Maine it is commemorated with a public holiday on the third Monday in April.
On Monday night, President Barack Obama noted the date, saying: ‘Today is Patriots’ Day – a day that reflects the freedom Boston has celebrated throughout its history.’
Counter-terrorism expert Richard Barrett said the incident had hints of a right-wing attack rather than Al Qaeda-inspired extremism.
Mr Barrett said the timing on Patriots’ Day and the relatively small size of the devices suggested the work of a domestic extremist.