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.US President Barack Obama, who will travel to Boston on Thursday for a memorial service, called the bombings an "act of terror" on Tuesday, while cities across the United States heightened security.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 were 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-colored nylon bag or backpack. The bag would have been heavy because of the components believed to be in it," DesLauriers said.Boston's WHDH television later showed a picture of an unattended, light-colored 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.US Homeland Security Secretary Janet Napolitano said there was no indication 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 bombAt 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.Much of central Boston remained cordoned off on Tuesday surrounding what Police Commissioner Ed Davis called "the most complex crime scene that we have dealt with in the history of our department."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 program 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."