The pressure-cooker type of explosive device is well-known to terrorism experts, having been used abroad and in a plot involving a planned attack in New York City.
Another law enforcement official said these types of explosives “most likely” would be triggered with timing devices and circuit boards rather than remote cellphones.
“We’re leaning towards timing devices,” he said, asking not to be identified because the investigation was underway. “With just 10 seconds between the two blasts, that might not have been enough time to set off both blasts with a cellphone.”
He also said that while pressure cookers are often used in the Middle East, they are also highlighted in bomb-making instructions available through the Internet in the United States.
In Boston, many of the dead and injured were sprayed with metal fragments, small nails and ball bearings, doctors and emergency responders told reporters. The bombs contained tiny pellets, similar to BBs, as well as other metal shards designed to make the weapons as deadly as possible.
Dr. Ron Walls, chairman of emergency medicine at Brigham and Women’s Hospital, said Tuesday that three of the hospital’s patients had shrapnel that appeared to include what he described as metal pellets about two to three millimeters in diameter. He said doctors also removed about a dozen small nails from one of the patients.
At Tufts Medical Center, doctors and nurses were cataloging all of the metal fragments removed from patients and turning it over to authorities, said Dr. William Mackey, chief of surgery. The shards ranged in size from a few millimeters to about a centimeter.
Dr. George Velmahos, chief of trauma surgery at Massachusetts General Hospital, said doctors there removed dozens of pellets, nails and other sharp objects from victims. He left open the possibility that some of the metallic fragments had been kicked up from the street but said that “most of them are in the bomb.”
“There are people that have 10, 20, 30, 40 in their body or more,” Velmahos said.
For the first time, federal officials -- led by President Obama -- publicly acknowledged that the explosions were acts of terrorism. The president will travel to Boston on Thursday morning to speak at an interfaith service dedicated to the victims of the attack.
“This was a heinous and cowardly act and, given what we now know about what took place, the FBI is investigating it as an act of terrorism,” Obama said in televised comments from the White House. “Any time bombs are used to target innocent civilians, it is an act of terror.
“What we don’t yet know, however, is who carried out this attack or why,” the president said, pledging to use all resources to find those responsible. “We will find whoever harmed our citizens and we will bring them to justice.”
“We also know this,” Obama said. “The American people refuse to be terrorized.”
“We will go to the ends of the Earth to identify the suspect or subjects responsible for this despicable crime,” DesLauriers said at an earlier news conference.
Meanwhile, two of the dead were identified -- a 29-year-old woman who had gone to the race with a friend to watch the friend’s boyfriend run the marathon; and an 8-year-old boy, Martin Richard, who was at the finish line with his family.
Martin’s father, Bill Richard, grieved along with neighbors and friends. Richard's wife and daughter were also wounded, and in a statement he called on everyone to pray for the family.
“My dear son Martin has died from injuries sustained in the attack on Boston,” his statement said.
“My wife and daughter are both recovering from serious injuries,” the statement continued. “We thank our family and friends, those we know and those we have never met, for their thoughts and prayers. I ask that you continue to pray for my family as we remember Martin. We also ask for your patience and for privacy as we work to simultaneously grieve and recover. Thank you.”
Neighbors and friends in the quiet Boston neighborhood where the boy had lived tried to come to terms with his death, bringing flowers and mementos to the family home, which police had surrounded with yellow tape by midmorning.
The Richards' neighbors in Dorchester told the Los Angeles Times that the boy loved to ride his bike and play with his older brother, Henry, and younger sister, Jane.
“Losing one child is bad enough, having the other ones injured and your wife injured,” said Jane Sherman, 64, who lives next door, trailing off. “They are a wonderful family and this is a horrific tragedy. I think this is something they won’t recover from.”
Friends and relatives of Krystle M. Campbell mourned her as the second victim of the Boston Marathon explosions.
“She was a wonderful person,” her mother, Patty, told reporters. “Everybody that knew her loved her. She loved her dogs … she had a heart of gold. She was always smiling. You couldn't ask for a better daughter. I can't believe this has happened. She was such a hard worker at everything she did. This doesn't make any sense.”
Krystle Campbell had worked in catering for several years with the Summer Shack restaurant group before moving up to manage their restaurant in Hingham, Mass. When that location closed a few months ago, she began to train as a manager at Jimmy’s Steer House, a steak restaurant in Arlington, Mass. The restaurant group’s director of operations, Nick Miminos, said he hired Campbell because she was an industry natural — a trained bartender who was knowledgeable about food and liquor costs, with impressive kitchen skills.
“She had one of those personalities that belongs in hospitality,” Miminos said. “She was instantly likable. The waitstaff loved working with her. She would run food for them, clear the tables for them. She wasn’t just a figurehead; she enjoyed getting her hands dirty.”
The third victim was a Boston University grad student, BU said in a statement. The student’s name has not been released, pending permission to do so from the family, the school said.
A jittery Boston awoke to heightened security a day after the attack. More than 400 members of the National Guard patrolled downtown. The adjacent blocks around Copley Square were blocked off with metal barricades and police tape, and many streets were closed to most traffic. Police and uniformed soldiers were allowing guests at nearby hotels -- some still in marathon gear -- to enter the restricted zone to retrieve their belongings from their rooms. Canine units were in the area.
“Everyone should expect continued heightened police presence, and everyone should continue personally to be vigilant,” Massachusetts Gov. Deval Patrick said at a news conference with top officials. “The investigation continues and until it is done, all of those in law enforcement represented by the leaders here will be present in force in the area around the blast and throughout the city.”
Patrick said no unexploded bombs were found at the Boston Marathon, contradicting earlier reports. Only the two bombs that exploded were found, he said.
“Yesterday, this terrorist brought to the city of Boston tragedy,” Mayor Thomas Menino said, then went on to praise first responders.
Runners were still trying to make sense of what they saw. For some, the explosions sounded celebratory, sounds one would hear at such an event. Nicholas Yanni, 32, a physics student, was with his wife near the finish line. He estimated that he was 10 feet away from a blast.
“It was like a cannon went off,” Yanni said. “At first I didn’t think anything of it.”
The blast shattered the window behind them and he saw smoke. He looked over at his wife, Lee Ann, 31, and saw that her lower leg was hit by shrapnel. She was bleeding, and he could see bone.
He was relatively OK – just ringing in his ears and muffled hearing from pierced eardrums. He was admitted to Tufts Medical Center but was expecting to leave the hospital Tuesday. Shrapnel shattered his wife’s fibula, he said, and she will need more surgery.
“It could have been so much worse for both us,” he told reporters at a news conference arranged by the hospital. Yanni said he understood the defiant attitude to take after attacks — not to let terrorists change their lives.
“When you’re actually right there where the bomb did go off, and you’re seeing stuff you just don’t really care to see, it’s kind of hard to say, ‘Yeah, I’ll go back … to a crowded event,’” Yanni said. “You’re always going to have that fear that something could wrong.”
Security was increased around the country, including in Los Angeles, New York and Washington. Police eyed commuters in subways, cordons of security around landmarks were extended and everyone was urged to report suspicious packages and people.
In Washington, a U.S. government official said that there had been no intelligence about a possible attack in Boston during the weeks leading up to the bombings.
No one has claimed responsibility, but at least one foreign group, the Pakistani Taliban, denied any role. In the past, the group has threatened attacks in the United States because of Washington's support for the Pakistani government.
The FBI did serve a warrant late Monday and searched an apartment in the Boston suburb of Revere. Some investigators were seen leaving the home early Tuesday carrying brown paper bags, plastic trash bags and a duffel bag.
At the Tuesday morning news conference, officials said Boston will recover, but the scars were still fresh. Inside a downtown-bound Red Line commuter train, riders were quieter than usual.
“Everyone's down,” said Leo Doolin, 49, of Dorchester, who was reading a newspaper article about the explosions. “Everybody's very quiet.”
Deanna Lewis, 21, a junior at Boston University, said people would just try to get on with their routines.
“Everyone has to go back to work or school,” she said as she studied a notebook while waiting for her train.
But she added: “There is still fear.”
PHOTOS: Bombings splashed across nation's front pages
Californians hurt in Boston Marathon blasts, including boy
Boston Marathon bombs: Crude, unsophisticated but still deadly
Dad of 8-year-old Boston bombing victim: 'Please pray for my family'
Bengali reported from Boston, Muskal from Los Angeles and Serrano from Washington. Times staff writers Alana Semuels, Andrew Tangel, Molly Hennessy-Fiske and Noam N. Levey contributed to this report from Boston. Brian Bennett and Michael A. Memoli in the Washington Bureau contributed to this report. Maeve Reston, Christine Mai-Duc, Alan Zarembo, Matt Pearce, Laura J. Nelson and Hailey Branson-Potts contributed from Los Angeles.