UPDATE 11:20 a.m.: Doctors say they removed a host of sharp objects from children and adults injured by the Boston Marathon explosions.
More than 170 people were hurt by the blasts, and doctors on Tuesday detailed some of the injuries, including broken bones, amputated limbs and head injuries.
"We've removed BBs and we've removed nails from kids. One of the sickest things for me was just to see nails sticking out of a little girl's body," said Dr. David Mooney, director of the trauma center at Boston Children's Hospital.
Bill Richard's son, 8-year-old Martin Richard, was killed in the attack. The father issued the following statement on Tuesday:
My dear son Martin has died from injuries sustained in the attack on Boston. My wife and daughter are both recovering from serious injuries. We thank our family and friends, those we know and those we have never met, for their thoughts and prayers. I ask that you continue to pray for my family as we remember Martin. We also ask for your patience and for privacy as we work to simultaneously grieve and recover. Thank you.
Two other children remain in critical condition at the hospital with serious leg injuries. Mooney said that tourniquets applied by emergency responders at the race saved the children's lives.
Doctors at Massachusetts General Hospital also say they removed metal fragments from victims of the two bombs.
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."
Massachusetts General treated 31 victims of the bombs. The hospital performed four amputations and at least two more patients have legs that are still at risk of amputation, Dr. George Velmahos said.
The father of a man who was photographed being pushed away from the Boston Marathon bombing in a wheelchair says his son has had both legs amputated.
Jeff Bauman says on this Facebook page that his son – 27-year-old Jeff Bauman Jr. – had to have both lower limbs removed at Boston Medical Center because of extensive vascular and bone damage. He says his son also had to have another surgery because of fluid in his abdomen.
Bauman says his son was there to watch his girlfriend run. She was not hurt. He says his son was in the wrong place at the wrong time.
Across the U.S., from Washington to Los Angeles, police tightened 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.
View President Barack Obama's latest statement on the bombings:
UPDATE 9:22 a.m.: President Barack Obama said Tuesday the deadly Boston Marathon bombings were an act of terrorism but investigators do not know if they were carried out by an international or domestic organization, or perhaps by a "malevolent individual."
In his second public statement in less than 24 hours since the explosions, the president said, "Clearly we are at the beginning of our investigation." He urged anyone with information relating to the events to contact authorities.
Individuals briefed on the probe said the two bombs were made up of pressure cookers, one packed with ball bearings and the other with shards of metal, presumably to inflict maximum injuries.
Obama said investigators "don't have a sense of motivation yet" as they begin to evaluate the attack in which three people were killed, including an 8-year-old boy, and more than 170 were wounded.
Despite the loss of life and limb, Obama declared, "The American people refuse to be terrorized."
As he had on Monday, he said those responsible for the attacks would be brought to justice.
The president had avoided labeling the incident a terrorist attack when he stood at the same White House lectern shortly after the explosions. Members of Congress quickly concluded on Monday afternoon that's what it was, and White House officials said the FBI was investigating the attack as a terror incident.
The administration's public assessment began to shift when Defense Secretary Chuck Hagel told Congress in a morning appearance that the attacks were "a cruel act of terror."
Appearing on television a short while afterward, Obama said the events in Boston were a "heinous cowardly act, and given what we now know about what took place, the FBI is investigating it as an act of terrorism."
"Any time bombs are used to target innocent civilians it is an act of terror. 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. That's what we don't yet know."
The president praised those who had come to the aid of the injured.
"If you want to know who we are, what America is, how we respond to evil, that's it: selflessly, compassionately, unafraid," he said.
Obama stepped to the microphone after receiving a briefing at the White House from Attorney General Eric Holder, Homeland Security Secretary Janet Napolitano and other top aides.
The bombs exploded on Monday afternoon near the finish line of the famed Boston Marathon, an annual 26 mile race through the neighborhoods of the city.
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."
Investigators also appealed to the public Tuesday for amateur video and photos that might yield clues to the Boston Marathon bombing as the chief FBI agent in Boston vowed "we will go to the ends of the Earth" to find whoever carried out the deadly attack.
UPDATE 6:57 a.m.: Massachusetts Gov. Deval Patrick says no unexploded bombs were found at the Boston Marathon. He says the only explosives were the ones that went off Monday.
They also reported that the number of injured has increased to 176, with 17 of them in critical condition and three dead.
The remarks came during a news conference Tuesday morning in Boston.
Three people were killed, including an 8-year-old boy, and more than 150 injured by two explosions just seconds apart near the finish line.
Special Agent in Charge Richard DesLauriers says there are no known additional threats and agents are following a number of leads.
ATF agent Gene Marquez says authorities are looking for amateur video and photographic evidence that can give clues to who set off the bombs.
In Southern California, law enforcement agencies are in a heightened state of alert. Officials from several agencies stress that there are no indications of a link between the Boston bombings and Southern California.
Still, you are likely on Tuesday to see more law enforcement personnel at places where large numbers of people congregate. And if you're travelling, there will likely be more officers with bomb-sniffing dogs patrolling area airports.
PREVIOUSLY: The bombs that blew up seconds apart at the finish line of one of the world's most storied races left the streets spattered with blood and glass, three dead, including an 8-year-old boy, more than 140 wounded and gaping questions of who chose to attack at the Boston Marathon and why.
Federal investigators said no one had claimed responsibility for the bombings on one of the city's most famous civic holidays, Patriots Day. But the blasts among the throngs of spectators raised fears of a terrorist attack.
President Barack Obama was careful not to use the words "terror" or "terrorism" as he spoke at the White House Monday after the deadly bombings, but an administration official said the bombings were being treated as an act of terrorism.
"We will find out who did this. We'll find out why they did this," the president said. "Any responsible individuals, any responsible groups, will feel the full weight of justice."
The FBI took charge of the investigation into the bombings, serving a warrant late Monday on a home in suburban Boston and appealing for any video, audio and still images taken by marathon spectators.
Some of the identities of the victims are being made public.
Martin Richard, 8, is being remembered as a vivacious boy who loved to run and climb. He was among the three people killed in the explosions Monday. That's according to multiple media reports and a person who talked to a friend of the family and spoke to The Associated Press on condition of anonymity. His mother and sister were badly injured.
A candle burned on the stoop of the family's single-family home in the city's Dorchester section Tuesday, and the word "Peace" was written in chalk on the front walkway.
Neighbor Betty Delorey says Martin loved to climb the neighborhood trees, and hop the fence outside his home.
The children's father, Bill, is the director of a local community group. The boy's mother, Denise, works at the Neighborhood House Charter School, where her children attend classes.
Meanwhile in Europe, a security official said Tuesday initial evidence indicates that the attacks were not the work of suicide bombers.
"So far, investigators believe it was not the work of suicide bombers, but it is still too early to rule it out completely," said the official, who spoke from the United States on condition of anonymity because he was not authorized to speak about the U.S. investigation.
The Pakistani Taliban, which has threatened attacks in the United States because of its support for the Pakistani government, on Tuesday denied any role in the marathon bombings.
The fiery explosions took place about 10 seconds and about 100 yards apart, knocking spectators and at least one runner off their feet, shattering windows and sending dense plumes of smoke rising over the street and through the fluttering national flags lining the route.
Blood stained the pavement, and huge shards were missing from window panes as high as three stories. Victims suffered broken bones, shrapnel wounds and ruptured eardrums.
Roupen Bastajian, a state trooper from Smithfield, R.I., had just finished the race when he heard the explosions.
"I started running toward the blast. And there were people all over the floor," he said. "We started grabbing tourniquets and started tying legs. A lot of people amputated. ... At least 25 to 30 people have at least one leg missing, or an ankle missing, or two legs missing."
At Massachusetts General Hospital, Alasdair Conn, chief of emergency services, said: "This is something I've never seen in my 25 years here ... this amount of carnage in the civilian population. This is what we expect from war."
Dr. Stephen Epstein of the Department of Emergency Medicine at Beth Israel Deaconess Medical Center says 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." He said it remained to be determined what exactly the objects were.
As many as two unexploded bombs were found near the end of the 26.2-mile course as part of what appeared to be a well-coordinated attack, but they were safely disarmed, according to a senior U.S. intelligence official, who also spoke on condition of anonymity because of the continuing investigation.
Massachusetts State Police said a search was conducted in the suburb of Revere on Monday night was related to the investigation, but provided no further details. Some investigators were seen leaving a building there early Tuesday carrying brown paper bags, plastic trash bags and a duffel bag.
Police said three people were killed. An 8-year-old boy was among the dead, according to a person who talked to a friend of the family and spoke on condition of anonymity. The person said the boy's mother and sister were also injured as they waited for his father to finish the race.
Hospitals reported at least 144 people injured, at least 17 of them critically. At least eight children were being treated at hospitals.
Tim Davey of Richmond, Va., was with his wife, Lisa, and children near a medical tent that had been set up to care for fatigued runners when the injured began arriving. "They just started bringing people in with no limbs," he said.
"Most everybody was conscious," Lisa Davey said. "They were very dazed."
The Boston Marathon is one of the world's oldest and most prestigious races and about 23,000 runners participated. The race honored the victims of the Newtown, Conn., shooting with a special mile marker in Monday's race.
Boston Athletic Association president Joanne Flaminio previously said there was "special significance" to the fact that the race is 26.2 miles long and 26 people died at Sandy Hook Elementary School.
One of the city's biggest annual events, the race winds up near Copley Square, not far from the landmark Prudential Center and the Boston Public Library. It is held on Patriots Day, which commemorates the first battles of the American Revolution, at Concord and Lexington in 1775.
Boston Police Commissioner Edward Davis asked people to stay indoors or go back to their hotel rooms and avoid crowds as bomb squads methodically checked parcels and bags left along the race route. He said investigators didn't know whether the bombs were hidden in mailboxes or trash cans.
He said authorities had received "no specific intelligence that anything was going to happen" at the race.
The Federal Aviation Administration barred low-flying aircraft within 3.5 miles of the site.
"We still don't know who did this or why," Obama said at the White House, adding, "Make no mistake: We will get to the bottom of this."
With scant official information to guide them, members of Congress said there was little or no doubt it was an act of terrorism.
"We just don't know whether it's foreign or domestic," said Rep. Michael McCaul, R-Texas, chairman of the House Committee on Homeland Security.
A few miles away from the finish line and around the same time, a fire broke out at the John F. Kennedy Library. The police commissioner said that it may have been caused by an incendiary device but that it was not clear whether it was related to the bombings.
The first explosion occurred on the north side of Boylston Street, just before the finish line, and some people initially thought it was a celebratory cannon blast.
When the second bomb went off, spectators' cheers turned to screams. As sirens blared, emergency workers and National Guardsmen who had been assigned to the race for crowd control began climbing over and tearing down temporary fences to get to the blast site.
The bombings occurred about four hours into the race and two hours after the men's winner crossed the finish line. By that point, more than 17,000 of the athletes had finished the marathon, but thousands more were still running.
The attack may have been timed for maximum carnage: 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.
Runners in the medical tent for treatment of dehydration or other race-related ills were pushed out to make room for victims of the bombing.
A woman who was a few feet from the second bomb, Brighid Wall, 35, of Duxbury, said that when it exploded, runners and spectators froze, unsure of what to do. Her husband threw their children to the ground, lay on top of them and another man lay on top of them and said, "Don't get up, don't get up."
After a minute or so without another explosion, Wall said, she and her family headed to a Starbucks and out the back door through an alley. Around them, the windows of the bars and restaurants were blown out.
She said she saw six to eight people bleeding profusely, including one man who was kneeling, dazed, with blood trickling down his head. Another person was on the ground covered in blood and not moving.
"My ears are zinging. Their ears are zinging," Wall said. "It was so forceful. It knocked us to the ground."