(CNN) -- Beyond the shattered glass, the blood, the wails of pain, there are the questions: Who did this? Why? And how?
Monday's terror attack on the Boston Marathon killed an 8-year-old boy, a young woman and one other person and left more than 170 others wounded. As doctors cared for the survivors of the twin bomb blasts, America mourned the senseless loss of three lives while the FBI's top investigator vowed to travel "to the ends of the Earth," if need be, to find those responsible.
For some, the attacks brought immediate comparisons to the September 11, 2001, attacks, partly because of reports that investigators were questioning Saudi Arabian citizens.
For others, the bombs that exploded near the finish line of the marathon Monday afternoon felt more homegrown, more akin to the April 1995 Oklahoma City bombing that left 168 people dead or the 1996 Atlanta Olympics bombing that killed two.
Authorities took pains to caution Americans against jumping to conclusions.
Combing through the debris, fanning out to interview witnesses, scanning radical websites, they were ruling nothing out -- or in -- on Tuesday.
President Barack Obama, speaking Tuesday after meeting with his national security team, seemed to signal that investigators are starting from scratch.
They don't know, he said, whether the bombing was a work of an organized group or a disgruntled loner. Nor do they know whether the violence came from within or was imported from overseas.
"This was a heinous and cowardly act, and given what we now know about what took place, the FBI is investigating it as an act of terror," he said.
The blasts Monday happened in quick succession in Boston's historic Copley Square, near the row of international flags leading to the finish line. The pressure wave whipped the limp flags straight out, as if they were caught in a hurricane.
Some runners said they thought the first blast was from a celebratory cannon. Any such illusions were shattered when the second blast erupted, startling the exhausted runners out of their post-race daze.
"When the second one happened, it was very 9/11-ish," runner Tom Buesse said on CNN's "Starting Point" on Tuesday.
As details emerged about the type of explosives used in the attacks, so did the intimate details of the victims.
An image of 8-year-old Martin Richard shows the boy holding a bright blue sign with the word "Peace." On Monday, the boy and his family were watching the Boston Marathon near the finish line when two bombs exploded just off Copley Square in the heart of the city.
The grade-schooler was killed, authorities said.
Krystle Campbell, who graduated from Medford High School in Massachusetts in 2001, was also killed in Monday's attack. There were no immediate details about the third victim.
As is often the case in such incidents, rumors quickly began to circulate of other bombs. Massachusetts Gov. Deval Patrick and federal investigators dispelled those rumors Tuesday, saying the only known explosives were the two bombs that went off.
Jittery passengers on a flight from Boston to Chicago on Tuesday expressed concern over two Arabic-speaking passengers, who were removed from the plane.
And security scares disrupted activities at Boston's Logan International Airport, LaGuardia Airport in New York and a train station in Cleveland.
All were unfounded, authorities said.
In Boston, police urged residents to be patient with the extra security precautions in transit stations and elsewhere. Investigators checked vents and pipes of buildings near the site of the explosions, according to a law enforcement source on the scene.
Meanwhile, city leaders promised to emerge unbowed.
"Boston will overcome," Mayor Thomas Menino said.
Although investigators still don't know who was behind the bombing, they moved closer Tuesday to understanding the devices used.
A federal law enforcement official with knowledge of the investigation told CNN on Tuesday that the bombs were apparently placed in a metal pressure cooker hidden inside a backpack.
Another law enforcement official told CNN it was "likely but not certain" that the bombs were on a timer and not set off remotely by a cell phone.
Another federal law enforcement official said both bombs were small, and initial tests showed no C-4 or other high-grade explosive, suggesting that the packages used in the attack were crude.
What remained unclear was whether the bombs had been packed with shrapnel or whether the metal containers blew apart, hurling pieces into the bodies of nearby victims.
Some law enforcement sources said they did not believe that the bombs had been packed with ball bearings or other shrapnel. But Dr. Ron Walls, the emergency medicine chairman at Boston Brigham and Women's Hospital, said surgeons there had retrieved something like nails, as well as "consistent, metallic, perfectly round objects" slightly larger than a BB, from victims.
"There is no question that some of these devices were, some of these objects were planted in the device for the purpose of being exploded forward when the bomb went off," he said.
Search for answers
Authorities pleaded for patience with swarming investigators, who expect to occupy Copley Square for at least two days, snarling traffic and interrupting the lives and work of countless Bostonians.
They asked the public to submit photos and videos that could help identify a suspect or explain how someone was able to slip bombs undetected into what Police Commissioner Ed Davis described as one of the most photographed spots in America.
The FBI took charge of the investigation, and Boston Field Office Special Agent in Charge Richard DesLauriers vowed to go to the "ends of the Earth" to find out who was behind the bombing.
"Our mission is clear: to bring to justice those responsible for the marathon bombing," he said. "The American public wants answers; the citizens of the city of Boston and the commonwealth of Massachusetts want and deserve answers."
Late Monday, authorities began searching the apartment of a young Saudi man who was on a student visa and injured at the race in nearby Revere. He was found to have no connection to the attack. "He was just at the wrong place at the wrong time," a U.S. official told CNN.
A Saudi woman, a medical student who was also injured in the blast, has also been interviewed by investigators, according to a law enforcement source.
Davis said many people were being questioned.
Like "being in Iraq"
Nurse Jim Asaiante was stationed near the finish line, expecting to treat the usual ailments from runners: cramps and dehydration.
Suddenly, he found himself in a battlefield, with blood and debris everywhere.
"For me, it was just like going back to being in Iraq in 2006-2007," said Asaiante, an Army captain who served an 18-month tour.
"I heard the first IED, and I know there's never one. The bad guys always set up two or three," he said.
Dr. Albert Pendleton, an orthopedic surgeon who was helping staff the race's medical tent, said Tuesday that it was "basically like the bomb took out the legs of everybody."
"It was horrific," he said.
Several patients lost limbs, doctors said Tuesday in briefings.
Of the more than 170 people who were treated at hospitals, at least 17 were in critical condition and 41 in serious condition, according to hospital officials. At least 51 had gone home as of Tuesday afternoon, according to a CNN tally.
At least nine of the wounded were children, as was one of the dead: 8-year-old Martin Richard.
One blast knocked 78-year-old marathoner Bill Iffrig to the ground.
"The shock waves just hit my whole body, and my legs just started jittering around. I knew I was going down," he said.
Iffrig was not seriously injured. But trails of blood, severed arms and legs and other body parts littered the scene.
Never the same again
After Monday's tragedy, some wondered whether the spectacle of the Boston Marathon would ever happen again.
The race -- which draws more than 20,000 participants -- is the world's oldest annual marathon, dating to 1897.
It's a tradition that not only symbolizes the arrival of spring in Boston, it marks Patriots Day, which commemorates the day of the opening battle of the Revolutionary War.
"The Boston Marathon has endured two world wars and many other things," said Fred Treseler, who has helped train more than 3,000 athletes for the race.
"I am quite sure there will be a Boston Marathon next year. But for certain, the Boston Marathon has been changed forever."
The terror attack rang alarm bells across the country.
Authorities in New York and Los Angeles stepped up security. In London, authorities began reviewing measures for that city's upcoming marathon.