BOSTON -- The two bombs that exploded at the Boston Marathon finish line with force that left debris on rooftops, killed three and injured more than 170, were fashioned from pressure cookers packed with nails, scraps of metal and ball bearings, and hidden inside two duffel bags, authorities said.

As President Barack Obama called the bombings an "act of terrorism," more than 1,000 law enforcement officers worked the case, sifting through nearly 2,000 tips by Tuesday afternoon. Officials speaking at late afternoon news conferences indicated that they don't know who set off the bombs or why.

With no suspects, and no group or individual claiming responsibility, authorities in this deeply wounded city pleaded with the public for more information. "At this time, there is no claim of responsibility," said Richard DesLauriers, FBI special agent in charge of the Boston office. "The range of suspects and motives remains wide open. . . . Someone knows who did this," he said.

Meanwhile Tuesday, the FBI, Boston police, federal Bureau of Alcohol, Tobacco, Firearms and Explosives agents, and state police recovered forensic evidence at the two blast sites. Those items were sent to the FBI lab in Quantico, Va., to be reconstructed.

Special agent Steven Bartholomew, a spokesman for the ATF, said Tuesday afternoon the blasts were large and high.

"Some debris were on top of rooftops, embedded in buildings," he said.

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Investigators also found a circuit board believed to have been used as a detonator and black explosive powder, according to a federal law enforcement official who asked not to be named.

"Among items partially recovered are pieces of black nylon, which could be from a backpack; what appears to be fragments of BBs and nails, possibly contained in a pressure-cooker device," said DesLauriers. " . . .The bag would've been heavy because of the components believed to be in it."

Doctors from Massachusetts General Hospital, where 34 patients were admitted, said pellets and nails were removed from victims and that four had to undergo amputations. Doctors at Brigham and Women's Hospital said 31 patients were rushed there and 19 remain; 10 are in critical condition, including a 5-year-old. Another three are classified as "serious" and six are considered "fair."

Some patients had metal embedded under their skin, while others suffered fractures and destroyed muscles, requiring reconstructive surgery, doctors said.

Tuesday afternoon, the third person to die in the Boston Marathon bombings was identified as a graduate student at Boston University who had been watching the race with two friends near the finish line. "The student's name has not been released, pending permission to do so from the family," the university said on its website. The Shenyang Evening News, a state-run Chinese newspaper, identified the third victim as Lu Lingzi, but a Chinese consulate spokesman told Bloomberg News a student with this name was missing.

Martin Richard, an 8-year-old boy from Dorchester with a toothy grin, and Krystle Campbell, 29, of Medford, Mass., were also killed. Martin's mother and sister were seriously hurt.

It was announced late Tuesday that Obama will attend an interfaith memorial service for the victims Thursday.

A preliminary analysis of the physical evidence collected at the two bomb sites shows they're similar to ones used by insurgents in Iraq and Afghanistan, said a federal law enforcement source with knowledge of the investigation.

"The shrapnel, the simplicity of it, it's something right out of the Iraq War . . . a basic roadside bomb," the source told Newsday.

Al-Qaida has published instructions on how to make such a device using a pressure cooker in its propaganda magazines, officials said. The FBI and Department of Homeland Security in 2010 issued warnings about the practice.

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The bombings are the most significant terror attack -- or attempt -- since Pakistani immigrant Faisal Shahzad parked an explosives-heavy sport utility vehicle in Times Square on May 1, 2010. Alert street vendors spotted smoke spewing from his vehicle and police disabled the bomb -- which was also inside a pressure cooker. He was sentenced to life in prison.

The shock of the marathon bombings and the horrific human toll resonated deeply across Boston, a city whose mayor regards it as a big family rather than a metropolis. John Hess Jr., of Boston, said his sister, Kara, lost part of her leg in the blast.

"The metal just sliced through her," said Hess, who was watching the race. "I'm in shock. You don't think this will happen at home."

Police Tuesday scoured the 12-block crime scene surrounding the blast sites and are examining surveillance and other video shot in the area. Members of the state National Guard, their guns slung over their shoulders, were stationed outside hotels late Tuesday as Bostonians, shaken but undeterred, milled about.

FBI and DHS agents conducted an early morning raid of the Water's Edge apartment complex in the Revere neighborhood, questioning a Saudi national overnight at a hospital, sources told Newsday. The man -- here on a student visa -- had burns on his hands and was tackled by authorities shortly after the bombings as he ran from the scene. By late afternoon, officials said there was no "person of interest" in the case.

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The youngest known fatality, Martin Richard, 8, loved soccer and baseball. The word "Peace" was painted on his family's driveway; flowers and candles adorned the stairs.

"He was such a sweet little boy," said tearful neighbor Rhonda Marques. "This has devastated not just their family, but the entire city. He was just too young."

Massachusetts congressman Stephen Lynch said he has known Martin's parents for nearly 25 years -- they worked on his first campaign and the boy's mother once worked with Lynch's wife at a local nursing home.

"They heard the first blast," Lynch said. "It was very close and it shocked them. They were in the process of trying to actually get out into the street, away from the buildings, but the barrier . . . that was to keep them out of the street was a problem. And, that was when the second bomb went off."

The FBI interviewed the four surviving family members Monday, Lynch said. He said the daughter is "grievously wounded" and that family members are "trying to circle the wagons" around her.

Krystle Campbell's father, William Campbell Jr., 56, said in a phone interview that his daughter, a manager at Boston's Jimmy's Steer House, was watching the marathon with a female friend.

He said doctors initially confused the two women and told him and his wife that they were operating on his daughter at Massachusetts General. But when they took him to see her, he said, "That's not my daughter."

Shortly after, a detective showed up with a photo of Krystle, saying she had died. "She was incredible," the heartbroken dad said. "She was there for everyone, no matter what."

On CNN, Rep. Peter King (R-Seaford) said, "If one thing positive can come out of this horrible event, it should be to alert people that the war against terrorism isn't over."