The timing of the Boston Marathon explosions reveals that the bombs may have been intentionally set to detonate the highest number of runners - and spectators would be near the finish line.
More than 18,000 runners participated in the marathon, beginning at staggered start times between 9am and 10.40am on Monday.
Video of the explosion shows the first bomb detonated at 4 hours, 9 minutes and 43 seconds into the third wave of runner - about the time a high number of runners were crossing the finish line.
A graph of finish times from the 2012 marathon shows that the highest percentage of runners finish between the 3:40 and 4:40 mark.
Scroll down for video
Timed: The bombs detonated 4 hours and 10 minutes after the start of the final wave of the marathon, at a time when thousands of runners were approaching the finish line
Four hours, nine minutes, 43 seconds: The timer clock reveals the moment the first bomb detonated near the finish line for the Boston Marathon
It's unknown whether the culprit knew these details when planting the bombs.
However, the revelations suggest the bomber studied the marathon well enough to know that the most runners and spectators would be present about 2.50pm, when the twin explosions detonated.
Three people died in the blasts - an eight-year-old boy, a 29-year-old woman and a Chinese graduate student. At least 183 people were wounded - including 23 who are still in critical condition and several who lost legs in the blast.
More...
Is this the second Boston bomb just seconds before it exploded? Dramatic new images show a bag discarded at spectators' feet as FBI crime scene pictures reveal deadly shrapnel suggesting first device WAS a pressure cooker
Revealed: The woman who became symbol of bomb blast horror who was watching her mother complete the marathon... and how her sister lost a leg in the attack
How horrific moment of second Boston Marathon explosion looked to one runner as she neared the finish line with camera attached to her head
Because each wave contains several thousand runners, it can be several minutes before many runners even reach the start line.
Federal agents zeroed in Tuesday on how the Boston Marathon bombing was carried out - with kitchen pressure cookers packed with explosives, nails and other lethal shrapnel - but said they still didn't know who did it and why.
FBI release images of bomb used at Boston Marathon
Improvised: The bombs were reportedly made from pressure cookers packed with ball bearings and explosives
An intelligence bulletin issued to law enforcement and released late Tuesday included a picture of a mangled pressure cooker and a torn black bag the FBI said were part of a bomb.
The FBI and other law enforcement agencies repeatedly pleaded for members of the public to come forward with photos, videos or anything suspicious they might have seen or heard.
'The range of suspects and motives remains wide open,' Richard DesLauriers, FBI agent in charge in Boston, said at a news conference. He vowed to 'go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime.'
Officials found that the bombs in Boston consisted of explosives put in ordinary 1.6-gallon pressure cookers, one with shards of metal and ball bearings, the other with nails, according to a person close to the investigation who spoke on condition of anonymity because the probe was still going on. The bombs were stuffed into black duffel bags and left on the ground, the person said.DesLauriers confirmed that investigators had found pieces of black nylon from a bag or backpack and fragments of BBs and nails, possibly contained in a pressure cooker. He said the items were sent to the FBI laboratory at Quantico, Va., for analysis.