* No additional bombs found in Boston, other U.S. cities watchful
* Too soon to identify suspects or motive - officials
* 8-year-old boy among dead, at least 176 injured (Adds details on victims, response to bomb components)
A pressure cooker stuffed with gunpowder and shrapnel caused at least one of the blasts at the Boston Marathon that killed three people and injured 176 others in the worst attack on U.S. soil since Sept. 11, 2001, law enforcement sources said on Tuesday.
President Barack Obama called the two bombings at the marathon finish line an "act of terror" and police said parts of central Boston could be closed for days as they investigated the blasts that maimed several people who lost legs above the knee.
"When these kids came in ... they were just so badly hurt, just covered with singed hair and in so much pain, it was just gut-wrenching," said David Mooney, the director of the trauma program at Boston Children's Hospital. "Pulling nails out of a little girl's flesh is just awful."
At least 10 people had limbs amputated as a result of their injuries, officials at hospitals said.
"Some of them woke up today with no leg and they told me that they are happy to be alive. They told me they thought they would die as they saw the blood spilling out," George Velmahos, chief of trauma surgery at Massachusetts General Hospital, told reporters.
The decision to amputate was easy, he said: "We just completed the ugly job that the bomb did."
The youngest to die was an 8-year-old boy, Martin Richard, who lived in the city's Dorchester neighborhood. Outside the family's home, sympathizers created a makeshift memorial of flowers and "Peace" was written in chalk on the sidewalk. .
Officials identified a second person killed as Krystle Campbell, 29, of Medford, Massachusetts. She was "the daughter that every father dreams to have," said Medford Mayor Michael McGlynn, who said he had confirmed the death with Campbell's father, William Campbell.
"She had a great sense of humor and freckles and red hair that brought her right to her Irish roots," McGlynn said.
The identity of the third person killed in the attack has not been released by officials.
An early lead in the investigation and an apartment search ended with law enforcement sources saying a Saudi Arabian student injured in the blast was likely to be cleared of suspicion. Neighbors in the man's apartment complex, who asked not to be identified, said FBI agents took boxes from the building.
No one has been arrested, Boston Police Commissioner Ed Davis told reporters on Tuesday morning.
Later on Tuesday in Washington, U.S. Homeland Security Secretary Janet Napolitano said there was no indication that the bomb blasts were part of a broader plot.
Officials in Britain and Spain said the London and Madrid marathons would go ahead on Sunday, but security plans for both races were under review.
PRESSURE COOKER BOMB
At least one bomb and possibly both were built using pressure cookers as the superstructure, black powder or gunpowder as the explosive and ball bearings as additional shrapnel, according to current and former counterterrorism officials briefed on the matter.
The sources, who asked not to be identified, said instructions on how to design such bombs are available on the internet.
"Any time bombs are used to target innocent civilians, it is an act of terror," Obama said in the White House briefing room. "What we don't yet know, however, is who carried out this attack or why, whether it was planned and executed by a terrorist organization - foreign or domestic - or was the act of a malevolent individual."
Officials in cities across the United States were watchful and several had security scares on Tuesday, although no injuries were reported.
Dispelling earlier reports of as many as seven devices being found around Boston, Gene Marquez, assistant special agent in charge for the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives, said authorities had determined that the only bombs deployed in the attack were the two that detonated shortly before 3 p.m. EDT (1900 GMT) on Monday.
Any unexploded device might have provided a clearer picture of what materials were used and how the bomb was assembled, furnishing leads in the case.
Trauma surgeons at several Boston hospitals told reporters a number of victims had metallic shrapnel material removed during surgery, including pellets and what appeared to be nails, which would make the device reminiscent of the 1996 bombing at the Summer Olympics in Atlanta, which killed two people and injured about 150 others. Anti-abortion activist Eric Rudolph, who eluded capture for years, pleaded guilty to that attack and is serving consecutive life sentences.