The bombs that exploded at the Boston Marathon were likely heavy, carried to the scene in dark nylon bags, and fashioned out of pressure cookers packed with shrapnel to make them more lethal, authorities said on Tuesday.

The twin blasts on Monday killed three people including an 8-year-old boy and injured 176 others, some of whom were maimed by bombs packed with ball bearings and nails. Seventeen victims remained in critical condition, and investigators still did not know who to blame.

President Barack Obama, who will travel to Boston on Thursday for a memorial service, on Tuesday called the bombings an "act of terror." It was the worst attack on US soil since security was stepped up following the suicide hijack attacks of September 11, 2001.

Cities across the United States went on heightened alert.

In Washington, the US Senate shut its mail facility for the next two to three days after Republican Senator Roger Wicker of Mississippi received mail that tested positive for the poison ricin.

In New York, bomb squad investigators were called in and the central terminal at La Guardia airport was evacuated due to a suspicious package. Two passengers and their bags were removed from a flight at Boston's Logan airport. Both cases turned out to be false alarms.

At the scene of the marathon, investigators recovered material that was being reconstructed at the FBI laboratory in Quantico, Virginia, said Richard DesLauriers, the Federal Bureau of Investigation's special agent in charge for Boston.

"Among items partially recovered are pieces of black nylon which could be from a backpack and what appear to be fragments of BBs (ball bearings) and nails possibly contained in a pressure cooker device," DesLauriers told a news conference.

"This morning it was determined that both of the explosives were placed in a dark-coloured nylon bag or backpack. The bag would have been heavy because of the components believed to be in it," DesLauriers said.

Bomb scene pictures taken by investigators and released on Tuesday show the remains of an explosive device including twisted pieces of a metal container, wires, a battery and what appears to be a small circuit board.

One picture shows a few inches of charred wire attached to a small box, and another depicts a half-inch (1.3 cm) nail and a zipper head stained with blood. Another shows a Tenergy-brand battery attached to black and red wires through a broken plastic cap. Several photos show a twisted metal lid with bolts.

A US government official, who declined to be identified, made the pictures available to Reuters.

In addition, Boston's WHDH television showed a picture of an unattended, light-coloured bag on the ground right at one of the bomb sites before the explosion. The bag was gone in a picture from a similar angle taken after the blasts. Authorities could not be reached for comment on the significance of the pictures.

No suspects were in custody and there were no claims of responsibility. "The range of suspects and motives remains wide open," DesLauriers said.

PRESSURE COOKER BOMB

At least one bomb and possibly both were built using pressure cookers as the container, black powder or gunpowder as the explosive and ball bearings as additional shrapnel, according to current and former counterterrorism officials briefed on the matter.

Doctors said some victims would have to endure several operations over the coming days.

"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 programme at Boston Children's Hospital. "Pulling nails out of a little girl's flesh is just awful."

Another doctor said he was amazed by the resolve of the patients.

"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."

DEAD AT 8 YEARS OLD

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, sympathisers 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 third fatal victim was a Chinese citizen whose identity was not being made public at the request of the victim's family, the Chinese Consulate in New York said in a statement. The victim was a graduate student at Boston University, the university said in a statement. - Reuters