BOSTON -- The explosives that killed three people and injured more than 170 during the Boston Marathon on Monday were most likely rudimentary devices made from ordinary kitchen pressure cookers, except that they were rigged to shoot sharp bits of shrapnel into anyone within reach of their blast and maim them severely, law enforcement officials said Tuesday.
The pressure cookers were filled with nails, ball bearings and black powder, and the devices were triggered by "kitchen-type" egg timers, one official said.
The resulting explosions sent metal tearing through skin and muscle, destroying the lower limbs of some victims who had only shreds of tissue holding parts of their legs together when they arrived at the Massachusetts General Hospital emergency room, doctors there said.
Law enforcement officials said the devices were probably hidden inside dark nylon duffel bags or backpacks and left on the street or sidewalk near the finish line. Forensic experts said the design and components of the homemade devices were generic, but that the marking "6L," indicating a six-liter container, could help identify a brand and manufacturer and possibly lead to information on a purchase.
New details about the explosives emerged as President Barack Obama announced that the FBI was investigating the attack as "an act of terrorism," and made plans to come to Boston on Thursday for an interfaith service at the Cathedral of the Holy Cross.
But officials said they still had no suspects in custody and did not give the impression that they were close to making an arrest, as they repeatedly noted that the investigation was in its infancy.
"The range of suspects and motives remains wide open," Richard DesLauriers, special agent in charge of the FBI's Boston office, said at a televised briefing Tuesday afternoon. And no one has claimed responsibility.
At this stage of an investigation, officials said it was not unusual for there to be no suspects. But with the paucity of leads, Mr. DesLauriers and others pleaded with members of the public to submit any photographs or video that they may have taken at the blast site to help in the investigation. At the briefing, Mr. DesLauriers said someone somewhere almost certainly heard someone mention something about the marathon or the date of April 15. "Someone knows who did this," he said. "Cooperation from the community will play a crucial role."
Officials said that as of Tuesday afternoon, they had received more than 2,000 tips from around the world. As marathoners left Tuesday through Boston's Logan Airport, security personnel reminded them of the importance of sharing their pictures with the FBI.
Counterterrorism specialists said authorities would aim to match the faces of possible suspects, using facial recognition software, against an array of databases for visas, passports and drivers' licenses. "It's our intention to go through every frame of every video that we have to determine exactly who was in the area," Boston police Commissioner Edward Davis said at the news briefing. "This was probably one of the most well-photographed areas in the country yesterday."
Among the three dead was an 8-year-old boy, Martin Richard of Dorchester. The boy had been watching near the finish line and then moved back into the crowd; the blast killed him and severely injured his mother and his sister.
Another spectator, Krystle Campbell, 29, of Arlington, Mass., also died Monday from injuries she suffered while watching the marathon, her grandmother Lillian Campbell said Tuesday.
The third person who died was identified by Boston University officials as a graduate student there, and the Chinese Consulate in New York said she was a Chinese national. The university is awaiting permission from the family before releasing her name. The young woman was watching the race close to the finish line, university president Robert Brown said in an email to the university community.
Given the force of the blasts, doctors at area hospitals said the death toll could have been much higher, but that the triage teams at the blast site had done a good job of parceling out the victims according to the capacity of each hospital to handle them. "The distribution worked wonderfully," said Stephen K. Epstein, attending emergency physician at Beth Israel Deaconess. "It was very easy to match the number of patients to the resources available at each at of the hospitals."
Boston is home to some of the country's most renowned medical institutions. Massachusetts General doctors said none of the hospitals was overwhelmed, allowing victims to be attended to in rapid order and saving lives in the process. Some victims were wounded so badly that even a delay of a few minutes could have meant they would have died, doctors said.
The scale of the attack and the crude nature of the explosives, coupled with the lack of anyone claiming to have been the perpetrator, suggested to experts that the attacker could be an individual or a small group rather than an established terrorist organization.
"This could have been a one-person job," said Georgetown University terrorism specialist Bruce Hoffman. "That makes it much harder to track. When we catch terrorists, it's usually because they're part of a conspiracy, and they're communicating with one another."
Nonetheless, a senior law enforcement official said authorities were also looking into connections between pressure cookers and al-Qaida in the Arabian Peninsula, the al-Qaida franchise in Yemen, largely because the design of the explosive device was described in a 2010 issue of the group's online English magazine, "Inspire."
"The pressurized cooker is the most effective method," the article said. "Glue the shrapnel to the inside of the pressurized cooker." The article was titled "Make a Bomb in the Kitchen of Your Mom."
Pressure cookers are designed to contain high pressure in order to cook food faster. Pressure cooker bombs work when explosive powder is set off inside the pot, and the resulting pressure builds until it exceeds the ability of the pot to contain it, creating a blast of tremendous force. Rudimentary explosive devices made from pressure cookers have been widely used in attacks in Afghanistan, India, Nepal and Pakistan, all countries where the cooking device is common, according to a Department of Homeland Security warning notice issued in 2010.
But they have occasionally turned up in attacks in the United States as well: Faisal Shahzad, a U.S. citizen who attempted a car-bomb attack on Times Square in May 2010, included a pressure cooker loaded with 120 firecrackers in the collective of improvised explosives in his SUV. The devices smoked but never exploded.
Instructions for assembling them can be found in many places on the Web, including in terrorism "cookbooks" popular among domestic extremists, and the al-Qaida magazine is easily available on the Internet. So the design did not necessarily point to a foreign connection.
A law enforcement official said the pressure cooker in Boston "was badly damaged," adding, "you can barely tell what it was." But enough of it remained intact to identify it, the official said.
One brand of pressure cooker with "6L" etched on the bottom is made by the Spanish company Fagor, which, according to its website, is the fifth-largest appliance manufacturer in Europe with factories in six nations, including Spain, China and Morocco, and subsidiaries in nearly a dozen more. The company sells about 50,000 of the six-liter pots in the United States every year, according to Sara de la Hera, vice president of sales and marketing at Fagor's U.S. subsidiary. Ms. De la Hera said she was unaware of whether the company had been contacted by investigators. It could not be immediately determined whether any other brand of pressure cooker also has "6L" etched on the bottom.
"It will have to go through ... many tests to see what they can glean further and identify where it was produced and sold, and then look at it forensically," the official added. Officials said Tuesday that evidence from the scene was being shipped to labs in Quantico, Va.
Bureau of Alcohol, Tobacco, Firearms and Explosives spokesman Steven Bartholomew said the blast was powerful enough to toss debris on top of buildings. "Some of that debris got projected on top of buildings, and embedded in buildings in that finish line area, so that tells us we have a lot of work to do," he said.
In Boston, as Tuesday wore on, many runners, clad in blue and gold jackets, made pilgrimages to the police blockade on Boylston Street, pausing to take pictures with their cell phones. Others came wearing jackets from previous marathons -- a symbol of accomplishment that in Boston turned into a sign of solidarity.
Bonnie Yesian was among many visitors still marooned in the city, because her hotel -- and her luggage and identification -- are inside the crime scene.
"I can't fly, so I'm stuck," said Ms. Yesian, who added that strangers and marathon volunteers had offered her guest rooms and supplies in the meantime.
Lauren Field, an auctioneer who now lives in Hampstead, N.H., was running in the race but was stopped blocks from the finish line. "It's heartbreaking to not cross the finish line; you train so hard for this," she said. "It's sad, but I'm safe."