BOSTON // Runners were still crossing the finish line four hours into the Boston Marathon on a perfect spring day when two blasts ripped through a crowd of onlookers.
The explosions sent flames and white smoke into the clear sky, and left dozens of people lying on the footpath amid limbs, blood and broken glass.
Yesterday morning, the area around Boylston Street where the blasts occurred remained cordoned off, the ground still strewn with crushed paper cups and overturned barriers, and a banner marking the finish line still fluttering over the deserted street.
Investigators had still not established a motive or named any suspects the day after two bombs exploded about 10 seconds and 90 metres apart, killing three and injuring at least 176 people.
Among those three was eight-year-old Boston boy Martin Richard.
Blood stained the pavement and huge shards were missing from window panes as high as three storeys.
Victims had broken bones, shrapnel wounds and ruptured eardrums.
At Massachusetts General Hospital, Alasdair Conn, chief of emergency services, said: "This is something I've never seen in my 25 years here … this amount of carnage in the civilian population. This is what we expect from war."
The bombings were the deadliest incidence of terrorism in the US since the tightening of security after the attacks of September 11, 2001, and underlined the impossibility of completely preventing such acts.
David Abel, a reporter for The Boston Globe, was taking video at the finish line when the bombs detonated.
"A lot of people were thinking, as often is the case with explosions, that it was a firework, perhaps - a celebratory gun salute for the finishing runners," Abel told NBC News.
"When I heard the second explosion, we knew immediately, like when we heard the plane hit the second tower of the World Trade Centre, that this was premeditated, that this was a terrorist attack."
A city source said the explosives were inside six-litre pressure cookers that were placed in black duffel bags. The source said the bombs were placed on the ground and contained shards of metal, nails and ball bearings.
Police asked for anyone with video or photographs of the attacks and their aftermath to hand them over.
They said they would scour security footage from dozens of nearby cameras frame by frame for evidence.
Police also searched a flat in the suburb of Revere, thought to be the residence of a Saudi Arabian student who was injured in the attacks and had been questioned in hospital.
The student was reportedly tackled by an onlooker and detained as he ran away from the explosions.
But yesterday morning investigators said they had determined the man was not involved in the attack.
The president, Barack Obama, said yesterday the bombings were an act of terrorism but investigators did not know if they were carried out by an international or domestic organisation, or a "malevolent individual".
With about 23,000 runners from 96 countries and more than a half million spectators, the world's oldest and most prestigious annual long-distance running event offered a highly visible, symbolic target for terrorists - and one that was almost impossible to secure. While those responsible are still unknown, there were many theories on the motive and the significance of the date yesterday. Monday was Patriots' Day, a public holiday in Massachusetts, and tax day in the US.
It also came shortly before the anniversaries of a number of other mass-casualty events: the Oklahoma City bombing on April 19; the Columbine High School shooting on April 20; and the Virginia Tech shooting, six years ago yesterday.
These dates could suggest the involvement of anti-government or white supremacist terrorists, but the White House and investigators said it was too early to link the attack to any suspect or cause.
But since Mr Obama came to office there has been a sharp rise in the number of anti-government "Patriot" groups.
A report by the Combating Terrorism Centre at West Point this year said "there has been a dramatic rise in the number of attacks and violent plots originating from individuals and groups who self-identify with the far-right of American politics".
The centre's director of terrorism studies, Arie Perliger, warned that there has been far too little attention paid to right-wing terror threats because of the focus on Muslims.
On Monday, suspicion immediately fell on Muslims before any evidence was found to support those claims.
Former Democratic Congresswoman Jane Harman spoke about the likelihood of Al Qaeda's involvement because the crude bombs could have been based on instructions published in the group's online magazine.
Erik Rush, a frequent conservative commentator on Fox News, went as far as to post on his Twitter account, "Let's bring more Saudis in without screening them!", and, "Yes, they're evil. Let's kill them all."
Haitham Al Salama, president of the Arab Caucus at the Harvard Kennedy School, called for responsible reporting on the events of Monday.
"In the aftermath of such event, and being a brown man, I couldn't help but feel for all brown men and women walking the streets in the US today, and in the few days or months to come," Mr Al Salama said. "For that, I encourage the media, American or otherwise, to be responsible and not help spread the wrong message."
Among those who witnessed in horror the carnage at the finish line were parents of the children murdered in the school shooting in Newtown, Connecticut, who were sitting in the VIP section across from the explosions.
A theme for this year's race was "26 Miles for 26 Victims", referring to the number of fatalities in Newtown.
* Taimur Khan was reporting from Washington. Additional reporting from AP