Investigators in protective suits examine the scene of one of the bomb blasts… (Elise Amendola, Associated…)
BOSTON — The bombs that tore past the finish line of the Boston Marathon were probably made with simple kitchen pressure cookers packed with metal pellets and nails and hidden in black nylon bags, investigators said Tuesday.
FBI lead investigator Rick DesLauriers said fragments from a pressure cooker and pieces of black nylon were discovered near one of the bomb sites.
Physicians said they had extracted from the wounded large numbers of pellets and carpenters' nails, common shrapnel components in the elementary bombs widely used in Afghanistan and Pakistan — and also discovered in at least two previous attempted terrorist attacks in the United States.
Photos: Explosions at Boston Marathon
The devices killed three bystanders and wounded more than 170 Monday and were powerful enough that investigators were recovering debris from nearby rooftops and embedded in walls.
"That gives you an idea of the scope, of the power of the blast, and you can see why it was so devastating," said Gene Marquez, acting special agent in charge of the Bureau of Alcohol, Tobacco, Firearms and Explosives in Boston.
Authorities have started tracing the source of the components to determine whether the perpetrators were foreign or domestic.
On Tuesday, the Pakistani Taliban denied responsibility, and no one else stepped forward.
President Obama, who was scheduled to travel to Boston on Thursday for an interfaith service for the victims, pledged to identify those responsible for what he called "an act of terrorism."
"Anytime bombs are used to target innocent civilians, it is an act of terror," Obama said. "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."
Call logs from cellphone towers along the marathon route and surveillance video collected by city cameras, local businesses and spectators — more than 10 terabytes of data, the equivalent of 6,000 movies — are being methodically reviewed, authorities said.
"This is the most complex crime scene that we've dealt with in the history of our department," said Boston Police Commissioner Edward Davis.
Boston remained tense Tuesday, with 12 blocks near Copley Square cordoned off. Many tried to return to work, soak up sunshine in sidewalk cafes and otherwise resume routines — on a day that was anything but normal.
"It's really hard to find the motivation to work," said Jesse Meyer, 46, an investment accountant at Liberty Mutual, whose office is a few blocks from the marathon finish line.
"We're just trying to continue on with our daily lives. Everyone has to go back to work or school," said Deanna Lewis, 21, a junior at Boston University. On the other hand, she said, "there is still fear."
Families confirmed the identities of two of those killed in the explosions: 8-year-old Martin Richard of Dorchester, Mass., whose mother and sister were also badly injured; and Krystle Campbell, 29, a restaurant worker from the Boston suburb of Arlington.
Also killed was a graduate student at Boston University, identified by the Chinese Consulate as a Chinese national, whose name was not released.
A picture of Martin in a school classroom, displaying a gap-toothed smile and holding a sign that says, "No more hurting people. Peace," quickly became one of the enduring images of the tragedy.
So did Campbell's mother, Patty, whose broken voice at a brief news conference could barely say she was "heartbroken."
"This doesn't make any sense," she said, before friends and family helped her back into the house.
At a candlelight vigil Tuesday night, hundreds of residents in tight-knit Dorchester gathered at a local park and reflected on the randomness of the deaths and injuries.
"It could be anyone," said Diane Lescinskas, whose daughters attended Catholic classes with Martin.
At hospitals throughout the day, families of survivors and physicians recounted the scenes that continued to haunt them: A zipper was found embedded in one woman's ankle; another woman's clothing was melted into her skin; a husband noticed his wife's legs were hanging on only by skin, and asked someone for a belt.
Surgeons told of wheeling patients directly into the operating room, after they had lost so much blood that their organs were in danger of failing, and amputating what was left of their legs on the spot.
"We just finished the job that the bomb did — their limbs were completely mangled, some hanging by a shred," said George Velmahos, chief trauma surgeon at Massachusetts General Hospital.
Some patients had large numbers of "nails or [other] sharp objects" embedded in them that surgeons worked painstakingly to excise. "There are people that have 10, 20, 30, 40 in their body or more," Velmahos said.
Investigators emphasized that their inquiry could be lengthy.