Media playback is unsupported on your device Media caption The blasts left three people dead and more than 170 injured

US President Barack Obama has condemned the twin bombing at the Boston Marathon as a "terrorist act".

He said the attack, near the finishing line, was "heinous and cowardly", but that the motive and culprit were not yet known and no-one was in custody.

Three people were killed, including an eight-year-old boy, and more than 170 injured by the bombs.

The FBI, which is running the investigation, has said there are no "known additional threats" to Boston.

Massachusetts Governor Deval Patrick says an interfaith service will be held for the victims on Thursday morning, attended by the president.

The mood in America after the Boston bombings generally seems one of sadness and horror but not one of anger or ramped-up patriotism. This is not 9/11

In his first briefing after Monday's attacks, Mr Obama had refrained from referring to terror attacks - a choice of words that drew criticism from some quarters.

But on Tuesday he said: "Any time bombs are used to target innocent civilians it is an act of terrorism."

He stressed it was not yet known whether an organisation - either domestic or foreign - or a "malevolent individual" was responsible, nor what the motive might have been.

"Everything else at this point is speculation," he said.

"It will take time... but we will find whoever harmed our citizens and we will bring them to justice."

'Despicable crime'

The first explosion went off close to the finish line at about 14:50 local time (18:50 GMT) on Monday.

Media playback is unsupported on your device Media caption President Obama: "A heinous and cowardly act"

Seconds later, as rescuers were rushing to help the injured, another explosion went off nearby.

There were early reports that other suspect devices had been found, but speaking in Boston on Tuesday, Mr Patrick said it was "important to clarify that two, and only two, explosive devices were found yesterday".

Mr Patrick said all other suspect parcels had been examined and discounted.

The FBI said it had no advance warning of a specific threat to the marathon.

Richard DesLauriers, the agent in charge of the investigation, moved to reassure the public, saying there was no longer any "known imminent physical threat" to Boston.

Police had received "voluminous tips" from the public since the bombings, he said, urging people to co-operate with investigators.

Analysis If investigators have a good idea whom they are looking for, they are not letting on. Instead they are emphasising the need for methodical evidence gathering - asking the public for photos and film from the marathon and continuing to examine what are being described as fairly crude bombs. The fact that there were only two devices increases the chance the attack is the work of only one or a small number of individuals rather than a major organised, international conspiracy. What is less clear is whether this was domestic anti-government extremism - of the type seen in the Oklahoma City bombing in 1995 - or someone influenced by al-Qaeda. The victims and the country as a whole want to know who was behind this attack and what motivated them. That answer may open up further difficult debates, but for the moment the priority remains finding those responsible as soon as possible.

"We will go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime, and we will do everything we can to bring them to justice," Mr DesLauriers said.

Police want the public to send in any videos or photographs they may have from the day.

Timothy Alban of the Massachusetts State Police said: "There have to be hundreds if not thousands of photographs or videos or observations that were made down at that finish line.

"You might not think it's significant but it might have some value to this investigation."

The type of device used in the attack has not been made public, but doctors treating the wounded have said their injuries indicate that the bombs contained metal shards and other shrapnel.

A number of victims have had limbs amputated.

George Velmahos, chief of trauma surgery at Massachusetts General Hospital, where many of the wounded were taken, said some had blast wounds "as well as small metallic fragments that entered their body - pellets, shrapnel, nails".

Unnamed security sources have been quoted in US media as saying at least one of the bombs was placed inside a pressure cooker packed with ball bearings and nails. This has not been officially confirmed.

Victims named

Eight-year-old Martin Richard, from the Dorchester area of the city, has been named as one of those who died.

He was at the finishing line with his mother and sister, who were both seriously injured.

"They were looking in the crowd as the runners were coming to see if they could identify some of their friends when the bomb hit," Congressman Stephen Lynch, a friend of the Richard family, told Associated Press.

Image caption Eight-year-old Martin Richard (bottom right) was one of the three people killed

He said Martin's brother and his father, Bill, were also injured.

Bill Richard released a statement thanking friends and supporters. "I ask that you continue to pray for my family as we remember Martin," it said.

Flowers and tributes were being placed outside the family's home on Tuesday.

A second victim was named in US media as Krystle Campbell, a 29-year-old restaurant manager.

Her father released a statement saying the family was devastated, AP reports.

Some 23,000 runners were taking part in this year's Boston marathon, which was being watched by hundreds of thousands of spectators.

The marathon is held on Patriots' Day, a Massachusetts state holiday which commemorates the first battles of the American Revolution in 1775.

Thomas Grilk, executive director of the Boston Athletic Association, said the race was "an integral part of the fabric and history of our community" and would go ahead next year.

"We are committed to continuing that tradition with the running of the 118th Boston Marathon in 2014, he said.

The London Marathon - the next major international marathon - is to go ahead on Sunday, with police saying they have well-rehearsed security plans.

Organisers have said they will hold a 30-second silence at the start as a mark of respect.