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Donna Leinwand Leger, Kevin Johnson and Doug Stanglin, USA TODAY

BOSTON - A search of an apartment in the suburb of Revere Tuesday is related to a man viewed by police as a "lead" -- but not a suspect -- in the investigation of the horrific Boston Marathon bombings that killed at least 3 people and injured more than 170, according to a law enforcement official.

Police, along with FBI and Homeland Security agents, descended on the high-rise Water's Edge apartment, removing brown papers bags, trash bags and a duffel bag among other items.

The official identified the man as a Saudi national. The man, like many at the time of the explosions, was observed running from the scene and sought medical treatment. The law enforcement official has been briefed on the matter but is not authorized to comment publicly on the case.

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Others news organizations have reported that the individual, who was hospitalized with injuries, is a Saudi national in the United States on a valid student visa.

Massachusetts State Police confirmed that a search warrant was served, but declined to provide further details, the Associated Press said.

The search of the apartment is part of an expanding investigation that is also focusing on the type of devices used in the twin blasts for possible clues on who was behind the terror attack.

"This will be a worldwide investigation," FBI special agent Richard Deslauriers told reporters at mid-morning. "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."

Among the dead in Monday's attack is 8-year-old Martin Richard from Dorchester, whose mother and sister were also badly injured, the Boston Globe reported. Martin was the son of Bill Richard, a community leader from Dorchester's Ashmont neighborhood, officials told the newspaper.

Authorities have not released the identities of the other victims from the explosions at the finish line of one of the world's premier sporting events.

Boston police commissioner Ed Davis said there were at least 176 casualties, including three dead and 17 people in critical condition.

The authorities have also issued a bulletin for an unidentified man who was seen running from a restricted area. But even then it was unclear whether the person being sought was anything other than a witness.

Since Monday's explosions at the finish line of the Marathon in downtown Boston, a law enforcement official said investigators have interviewed "hundreds'' of witnesses in an attempt to identify the source of the attacks.

In addition, the FBI is asking residents to come forward with any photos or videos that could help in the investigation.

A law enforcement official said the explosive devices, which went off about about 12 seconds and 100 yards apart, were believed to be assembled with gunpowder and ball-bearing-type material to serve as shrapnel. The official, who has been briefed on the matter but is not authorized to comment publicly, described the devices as "rudimentary'' but powerful.

It was unclear whether the devices were remotely detonated or included timers, the official said, adding that no conclusions had been drawn on whether an organized group or lone wolf had been responsible for the attack.

Police have cordoned off a 12-block area around Boston's Copley Square and bolstered security around the city as they continue to gather evidence.

A European security official with knowledge of the Boston investigation said initial evidence indicates they were not detonated by suicide bombers.

The official spoke from the United States on condition of anonymity because he was not authorized to speak about the U.S. investigation. "Investigators believe it was not the work of suicide bombers" but it's still too early to know for sure, he said.

On Tuesday, meanwhile, Ehsanullah Ehsan, a spokesman for the Taliban in Pakistan, told the AFP news agency that it was not involved in the attack. The Pakistani Taliban has claimed responsibility for the 2010 Time Square bomb plot.

Massachusetts Gov. Deval Patrick said Tuesday that the two devices that exploded were the only ones involved in the attack. He said no unexploded devices were found.

Officials earlier said that as many as seven suspicious packages were destroyed in controlled explosions but they were later found not to be actual bombs.

The state, local and federal investigation into the bombings is being led by the FBI. Special Agent Rick Deslauriers said investigators were looking at possible terrorism, but he refused comment on specific suspects or leads in the case.

"It is a very active, fluid investigation at this time,'' he said.

The blasts ignited a fresh round of unease and renewed security concerns across the nation. The stark pictures of mayhem and the injured sent over TV and the Internet also rekindled stark memories from the 9/11 terrorist attacks on New York's World Trade Center and the Pentagon. Monday's attacks also come just four days before the anniversary of the April 19, 1995, bomb attack on Oklahoma City's Murrah Federal Building.

In the wake of the Boston attack, security has been increased around landmarks and prominent hotels in Washington and New York. Pedestrians have been excluded from walking in front of the White House as security officials extended the security zone to include Lafayette Park.

Speaking from the White House, a somber President Obama said people should not speculate over who was responsible.

"We still don't know who did this or why. People should not jump to conclusions before we have all the facts. But make no mistake. We will get to the bottom of this. We will find out who did this. We will find out why they did this. Any individual or responsible groups will feel the full weight of justice."

The Boston Marathon blasts occurred about three hours after Ethiopia's Lelisa Desisa won the 26.2-mile race.

Bloodied spectators were carried to a medical tent intended for runners. At least one police officer was hurt.

"There were so many people in that area that they couldn't get ambulances in there," said Joe Difazio, who was working on communications near the site when the blasts occurred. "They were wheeling people out in wheelchairs. One guy had no legs. The bones was just sticking out ... It was horrible."

Organizers stopped the race and locked down the marathon headquarters.

The Federal Aviation Administration announced a temporary flight restriction over Boston.

Cellphone service was operating, wireless companies reported, contrary to an earlier Associated Press report quoting a law enforcement official who said service was cut in case there were other undetected devices.

The elite women runners started the race at 9:30 a.m., and the elite men followed about 30 minutes later. About 27,000 runners were in the field for the Patriots' Day race.

Kimberly DelGuzzi of Pittsburgh was waiting on Boylston Street for her friend to cross the finish line when she found herself pressed against a building, ducking for cover from the blasts.

"At first, I thought it was fireworks, but then I saw the smoke go up in the air," said DelGuzzi, who was standing between the two explosions. "Then, not even a minute later, the second one went off."

She described the scene as "mass chaos" and said, "Oh my God, it was loud."

"The explosions shook everything," she said, her voice still shaking 40 minutes after the bombs went off. "I saw runners down in the street. I saw people down on the sidewalk."

Tom Beusse, president of the USA TODAY Sports Media Group, had just finished the race and was about 150 yards away from the explosion.

"There was this giant explosion. All of us turned around, the runners, and had these looks on their faces like 'Oh my God.' ... Immediately, it turned into mayhem. People were screaming. Cops told us to keep moving away from the finish line in the direction we were going. No one knew what was coming next."

Around the city, friends and bystanders gathered outside of hospitals to comfort one another. Marathoner Kirsten Scott was still wearing her number as she talked with a friend and thought about her husband, a surgery resident tending to patients. "We're just praying," she said.

Contributing: Donna Leinward Leger, Melanie Eversley, Gary Stoller and Susan Davis in Boston; Kevin Johnson, David Leon Moore, Gary Strauss, Liz Szabo, and Oren Dorell, USA TODAY