Martin Richard, from Dorchester, a neighbourhood of Boston, was among the three people killed in the blasts yesterday that erupted just metres from the marathon finish line.
A candle has been reportedly placed on the stoop of his family's home, while the word "Peace" has been written in chalk on the front walkway.
As the FBI launched a hunt for those responsible, US President Barack Obama said: "We will find out who did this. We'll find out why they did this. Any responsible individuals, any responsible groups, will feel the full weight of justice."
There were no immediate claims of responsibility for the attack, the most serious in the US since the 9/11 atrocity.
Betty Delorey, who lives nearby to Martin's family home, told reporters Martin loved to climb the neighbourhood trees and hop the fence outside his home.
His father, Bill, is the director of a local community group, while his mother Denise works at the Neighbourhood House Charter School.
Bill Bratton, a former head of Boston police who is now based in London, said: "Unfortunately in my country there are no shortage of potential suspects, if you will."
Meanwhile,
counter-terrorism expert Richard Barrett said the incident had hints of
a right-wing attack rather than al Qaida-inspired extremism.
Mr
Barrett, the former United Nations co-ordinator for the al Qaida and
Taliban monitoring team, said it was too early to say who was to blame
for the marathon blasts.
But Mr Barrett,
who has served with MI5 and MI6, said the timing on Patriots' Day and
the relatively small size of the devices suggested the work of a
domestic extremist.
"At the moment it
looks more likely that it was a right-wing terrorist incident, rather
than an al Qaida attack because of the size of the devices."
He
added: "This happened on Patriots' Day, it is also the day Americans
are supposed to have their taxes in, and Boston is quite a symbolic
city. These are all little indicators."
The
Pakistani Taliban, which has threatened attacks in the US because of
its support for the Pakistani government, denied any role in the
bombings.
Police were searching a flat in the Boston suburb of Revere as part of the investigation.
Massachusetts State Police confirmed that a search warrant was served last night but gave no more details.
Some investigators were seen leaving the property early today carrying brown paper bags, plastic binbags and a duffel bag.
The
Boston Marathon is one of the world's oldest and most prestigious and
about 23,000 runners took part, including hundreds from the UK and
Ireland.
The race honoured the victims of the Newtown, Connecticut, school shooting with a special mile marker.
Boston
Athletic Association president Joanne Flaminio previously said there
was "special significance" to the fact that the race is 26.2 miles long
and 26 people died at Sandy Hook Elementary School.
Scores
of victims were hurt, many suffering shrapnel wounds, in the two blasts
which rocked the finish line. Massachusetts General Hospital said
"several amputations" had to be performed on survivors.
Horrifying
images of injured spectators and blood-spattered pavements were caught
as television cameras broadcast footage of the explosions.
Hospitals reported at least 144 people injured, at least 17 of them critically.
The
eight-year-old boy who died was with his mother and sister, who were
also injured as they waited for his father to finish the race.
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."
Tim Davey, of
Richmond, Virginia, was with his wife, Lisa, and children near a medical
tent that had been set up to care for fatigued runners when the injured
began arriving.
"They just started bringing people in with no limbs," he said.
Ed Davis, the city's police commissioner, said there had been two "simultaneous explosions" near the finish line.
The first occurred just before the photo bridge that marks the finish line. The second could be heard a few seconds later.
They
occurred about four hours into the race, a time when large numbers of
runners would be finishing, with many more relatives and friends
cheering them on.
Security for the London Marathon on Sunday will be reviewed following the explosions.
But sports minister Hugh Robertson insisted he was "absolutely confident" that the London event could be kept safe.
He
said: "I think this is one of those incidents where the best way to
show solidarity with Boston is to continue and send a very clear message
to those responsible..."
London Mayor
Boris Johnson has spoken to Metropolitan Police Commissioner Sir Bernard
Hogan-Howe about the possibility of extra security for the London
Marathon.
Mr Johnson said: "These
attacks were aimed at its core, at innocent men, women and children
enjoying a spring day out at a major sporting event.
"We
do have robust security measures in place for Sunday's London Marathon
but, given events in Boston, it's only prudent for the police and the
organisers of Sunday's race to re-examine those security arrangements."
A
spokeswoman for the Home Office confirmed that Home Secretary Theresa
May had been fully briefed and held a meeting with MI5 director-general
Jonathan Evans and the Met's assistant commissioner for specialist
operations, Cressida Dick.
The current
threat level to the UK from international terrorism, set by the Joint
Terrorism Analysis Centre (JTAC), remains at substantial.
here
were 176 casualties in the bombings, including three fatalities and 17
people who remain in critical condition, a press conference in Boston
heard.
Democrat governor of
Massachussets Deval Patrick said: "It's important to clarify that two
and only two explosive devices were found yesterday.
"Other
parcels, all other parcels, in the area of the blast have been examined
but there are no unexploded bombs. There were no unexploded explosive
devices found.
"Over 150 people were
injured yesterday in the blast, some gravely. Our thoughts go out to all
of those injured and killed and to their family and friends."