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Boston - AP - One of the explosive devices used in the Boston Marathon attack appears to have been placed in a metal pressure cooker packed with nails and ball bearings, CBS News has learned, as authorities appealed to the public Tuesday for amateur video and photos that might yield clues to the bombing.

The details on the explosives emerged as the chief FBI agent in Boston vowed “we will go to the ends of the Earth” to find those responsible.

A law enforcement source told CBS News senior investigative producer Pat Milton that one of the explosive devices appears to have been placed in a metal pressure cooker (a metal kitchen pot with a locked-down top) which had been placed in a black nylon bag or backpack. Investigators also found pieces of an electronic circuit board possibly indicating a timer was used in the detonation of the bomb.

A law enforcement official told CBS News that the two bombs that exploded were made to look like discarded property. It is still unknown if one or both bombs were in garbage cans. One may have been on the sidewalk.

The bombs were described as “low explosive,” but with “anti-personnel” packing. The official said there were apparently things like BB’s, ball bearings and nails in the bombs. This is consistent with doctors reporting shrapnel pulled from victims.

A doctor treating the wounded said one of the victims was maimed by what looked like ball bearings or BBs. Doctors also said they removed a host of sharp objects from the victims, including nails that were sticking out of one little girl’s body.

At the White House, meanwhile, President Barack Obama said that the bombings were an act of terrorism but that investigators do not know if they were carried out by an international organization, a domestic group or a “malevolent individual.”

He added: “The American people refuse to be terrorized.”

Across the U.S., from Washington to Los Angeles, police stepped up security, monitoring landmarks, government buildings, transit hubs and sporting events. Security was especially tight in Boston, with bomb-sniffing dogs checking Amtrak passengers’ luggage at South Station and transit police patrolling with rifles.

“They can give me a cavity search right now and I’d be perfectly happy,” said Daniel Wood, a video producer from New York City who was waiting for a train.

Similar pressure-cooker explosives have been used in Afghanistan, India, Nepal and Pakistan, according to a July 2010 intelligence report by the FBI and Homeland Security. Also, one of the three devices used in the May 2010 Times Square attempted bombing was a pressure cooker, the report said.

“Placed carefully, such devices provide little or no indication of an impending attack,” the report said.

The Pakistani Taliban, which claimed responsibility for the 2010 attempt in Times Square, has denied any role in the Boston Marathon attack.

The two bombs blew up about 10 seconds and around 100 yards apart Monday near the finish line of the 26.2-mile race, tearing off limbs, knocking people off their feet and leaving the streets stained with blood and strewn with broken glass. The dead included an 8-year-old boy and a 29-year-old woman.

“We started grabbing tourniquets and started tying legs. A lot of people amputated,” said Roupen Bastajian, a state trooper from Smithfield, R.I., who had just finished the race when he heard the explosions.

Federal investigators said no one had claimed responsibility for the bombings, which took place at the world’s best-known distance race, held every year on one of Boston’s biggest holidays, Patriots’ Day.

“We will go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime, and we will do everything we can to bring them to justice,” said Richard DesLauriers, FBI agent in charge in Boston.

He said investigators had received “voluminous tips” and were interviewing witnesses and analyzing the crime scene.

Gov. Deval Patrick said that contrary to earlier reports, no unexploded bombs were found.

Boston police and firefighter unions announced a $50,000 reward for information leading to arrests in the bombing.

Law enforcement sources told CBS News’ Milton that a Saudi Arabian man who was being questioned by investigators is not being considered a suspect at this time, and it appears that he was a spectator who was injured in the attack.

At a news conference, police and federal agents repeatedly appealed for any video, audio and photos taken by marathon spectators, even images that people might not think are significant.

“There has to be hundreds, if not thousands, of photos and videos” that might help investigators, state police Col. Timothy Alben said.

Boston Police Commissioner Edward Davis said investigators also gathered a large number of surveillance tapes from businesses in the area and intend to go through the videos frame by frame.

“This is probably one of the most photographed areas in the country yesterday,” he said.

At least 17 people were critically injured, police said. At least eight children were being treated at hospitals. In addition to losing limbs, victims suffered broken bones, shrapnel wounds and ruptured eardrums.

Dr. Stephen Epstein of the emergency medicine department at Beth Israel Deaconess Medical Center said he saw an X-ray of one victim’s leg that had “what appears to be small, uniform, round objects throughout it — similar in the appearance to BBs.”

Dr. Richard Wolfe, chief of Emergency Medicine at Beth Israel Deaconess Medical Center, told CBS News Tuesday that the injuries sustained in the bombing have been primarily shrapnel injury in the lower extremities.

“Some hand injuries, but mainly devastating injuries to limbs,” Wolfe said. “We have at least two amputations and a number of very serious wounds that require fairly aggressive care.”

Eight-year-old Martin Richard was among the dead, said U.S. Rep. Stephen Lynch, a family friend. The boy’s mother, Denise, and 6-year-old sister, Jane, were badly injured. His brother and father were also watching the race but were not hurt.

Krystle Campbell, a 29-year-old restaurant manager, also died in the bombing, WBZ Radio confirmed Tuesday afternoon. William Campbell told the AP that his daughter had gone with her best friend to take a picture of the friend’s boyfriend crossing the finish line.

Neighbor Betty Delorey said Martin loved to climb neighborhood trees and hop the fence outside his home.

About 23,000 runners participated in this year’s Boston Marathon. Nearly two-thirds of them had crossed the finish line by the time the bombs exploded, but thousands more were still completing the course.

Demi Clark, a runner from North Carolina who said she was the crossing finish line as the first blast went off, told CBSNews.com “blood was everywhere instantly.”

“Nobody knew what to do - after the second one went off we were like, ‘the city’s under attack,’” Clark said.

The attack may have been timed for maximum bloodshed: The four-hour mark is typically a crowded time near the finish line because of the slow-but-steady recreational runners completing the race and because of all the friends and relatives clustered around to cheer them on.

Davis, the police commissioner, said authorities had received “no specific intelligence that anything was going to happen” at the race. On Tuesday, he said that two security sweeps of the route had been conducted beforehand.

Patriots’ Day commemorates the opening shots of the American Revolution, at Concord and Lexington in 1775.

Richard Barrett, the former U.N. coordinator for an al Qaeda and Taliban monitoring team who has also worked for British intelligence, said the relatively small size of the devices in Boston and the timing of the blasts suggest a domestic attack rather than an al Qaeda-inspired one.

“This happened on Patriots’ Day — it is also the day Americans are supposed to have their taxes in — and Boston is quite a symbolic city,” said Barrett, now senior director at the Qatar International Academy for Security Studies.