Authorities in the United States are on the hunt today for whoever is responsible for the bombings at the Boston Marathon, and said they are co-ordinating a "worldwide investigation."
"We will go to where the evidence and the leads take us," FBI special agent Rick DesLauriers.
"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," he said.
U.S. President Barack Obama said today the explosions are being investigated as an act of terror, although authorities still don't know who's responsible.
He called the bombings "a heinous and 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 attack innocent civilians it is" a terrorist act, he said.
Obama also said it's not clear yet whether the bombings were the work of a foreign or domestic group or a "malevolent individual."
He said investigators "don't have a sense of motivation yet," adding that "clearly we are at the beginning of our investigation."
He urged anyone with information related to the attacks to contact authorities and he warned people to be extra vigilant and to report any suspicious packages, parcels or activity.
Obama also declared "The American people refuse to be terrorized" adding "If you want to know who we are, what America is, how we respond to evil, that's it: selflessly, compassionately, unafraid."
Police are asking the public for any information - big or small - that might provide clues, such as amateur video or cell phone photos.
"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 have a large number of surveillance tapes from the area and are going through them frame by frame.
FBI agent DesLauriers said investigators had received "voluminous tips" and were interviewing witnesses and analyzing the crime scene.
Investigators are analyzing what's left of the bombs, which have been described in reports as relatively small and crude with no C-4 or other high-grade explosive.
The FBI wouldn't say anything specific about the bombs, where they might have been hidden or whether they were packed with shrapnel for maximum damage
However, a source told CNN investigators think the bombs may have been put in trash cans.
And the Associated Press cited a source who said the bombs were made out of pressure cookers and packed with shards of metal, nails and ball bearings.
Dr. George Velmahos, the head of the trauma department at Massachusetts General Hospital, said many wounds were caused by "small metallic fragments" such as pellets and nails.
But also other pieces of metal, including some that had "spiked points that resembled nails without heads."
Last night, authorities searched a 5th-floor apartment unit in the Boston suburb of Revere, but didn't give any other details.
Investigators left the building early today carrying brown paper bags, plastic trash bags and a duffel bag, according to The Associated Press.
Authorities also confirmed they haven't found any other explosive devices other than the two that went off at the marathon.
Police say the bombs went off within 10 seconds of each other, about 45 to 90 metres apart near the finish line - just over 4 hours into the race.
A former federal law enforcement source told CNN that could be significant as it "seems to indicate this was not geared toward maximum damage."
"It may speak volumes about the (level of) planning that went into this," the source said.
"It raises questions... why didn't the bombs go off when the crowd was packed in like sardines when the winners were crossing the finish line? It could mean the people behind it couldn't get access to the area when they originally intended."
Three people were killed, including an 8-year-old boy, who was watching his father race.
The boy has been identified as Martin Richard of Boston. His mother and sister were said to be critically wounded.
More than 170 others were hospitalized - at least 17 in critical condition and 41 in serious condition, with injuries that include amputations, lower leg and shrapnel wounds, and ruptured eardrums.
Dr. Albert Pendleton, an orthopedic surgeon who was in the race's medical tent, told CNN it was "basically like the bomb took out the legs of everybody."
Dr. Natalie Stavas, a pediatric doctor in Boston who was in the marathon, raced to the scene to help.
"I was a critical care nurse for quite some time and have worked in trauma emergency rooms, and none of that prepared me for the devastation and the horror that was at the finish line," she said.
Another nurse Jim Asaiante - a former U.S. army captain who served in Iraq - was working near the finish line.
"For me, it was just like going back to being in Iraq in 2006-2007," he said. "I heard the first IED, and I know there's never one. The bad guys always set up two or three."
At least nine of the wounded were children but officials say some have already left hospital.
Some runners said they thought the first explosion was from a cannon in celebration of the marathon. But after the second bomb, they knew different.
"When the second one happened, it was very 9-11ish," runner Tom Buesse told CNN today.
78-year-old runner Bill Iffrig, who was in his third Boston Marathon, was knocked to the ground by one of the explosions but wasn't seriously hurt.
"I was just approaching the last straightaway to the finish line, and I had a good day and was feeling really good, and I got down to within about 15 feet of the finishing apron and just tremendous explosion, sounded like a bomb went off right next to me," Iffrig said.
"The shock waves just hit my whole body and my legs just started jittering around. I knew I was going down," he said.
Another runner, Tami Hughes, had just crossed the finish line and was looking for her husband when the first bomb went off.
"I didn't know if it was a small aircraft going into the building," said Hughes.
"I turned around and immediately saw the whitish-brownish smoke billowing up four or five stories and I couldn't believe that, you know, could it be a bomb? And I stared at it and about five or seven seconds later, when the second one went off, I knew immediately that it was something coordinated or organized."
Runner Lisa Bentley told CBC News she heard both explosions and felt the ground shake.
"We didn't know what had happened at all - we just started seeing the lights and the sirens and the emergency vehicles and we thought we've just got to get away from here, so we kept walking away from the finish line," said Bentley, who had already finished the race.
Foreign Affairs officials told CBC News there haven't been any reports of Canadians injured in the attacks.