Boston Marathon bombing suspect Dzhokhar Tsarnaev remains at large after the death of his brother Tamerlan Tsarnaev, who is also suspected of carrying out the attacks.
Speculation is flooding social networks as more details come to light. Here is the chain of events following the Boston Marathon bombings.
Two explosions occur within 15 seconds of each other around four hours into the race. The attack may have been timed for maximum carnage, as the four-hour mark is typically a crowded time near the finish line.
Initial reports say two people have been killed but the death toll later goes up to three. Hundreds more are injured, many seriously.
US President Barack Obama says those responsible for the attacks will feel the "full weight of justice".
The FBI says no one has claimed responsibility for the bombings.
There are reports that a suspect has been taken into custody in connection but this is later denied by the authorities.
President Obama declares "every one of us stands with you" as he addresses an interfaith service to remember the victims at Boston's Cathedral of the Holy Cross.
Investigators are said to be focused on hunting a man seen on surveillance footage from a department store near the finish line, dropping off a bag and walking away from the site of the second of the two explosions.
5.30pm (10.30pm UK time) - The FBI releases CCTV footage and stills of two suspects from surveillance cameras near the explosion sites filmed shortly before the blasts.
FBI agent Richard DesLauriers asks for the public's help in identifying the men who are both wearing baseball caps and carrying backpacks. "We consider them to be armed and extremely dangerous," Mr DesLauriers said, warning that they should not be approached.
10.30pm (3.30am UK time) - A campus police officer is shot and killed at Massachusetts Institute of Technology (MIT) in Cambridge, near Boston, after responding to a report of a disturbance.
Shortly afterwards, police get a report of a carjacking nearby. The two men are suspected of killing the MIT police officer, then stealing the car at gunpoint and later releasing its driver unharmed. The suspects threw explosives from the car as police followed it to the Boston suburb of Watertown.
1am (6am UK time) - Witnesses report hearing multiple gunshots and explosions in Watertown. Residents are advised to keep their doors locked and not let anyone in. TV footage shows armed officers surrounding a suspect lying on the ground.
Police later say one of the two suspects in the MIT officer shooting is dead but the other, who is tied to the Boston marathon bombing, remains at large.
Public transport is suspended in the Boston area as the hunt for the remaining suspect continues.
A new photo of him on the loose is released showing him in a grey hoodie sweatshirt at a 7-Eleven store in Cambridge.
The two suspects are said to be from a Russian region near Chechnya and have lived in the US for at least one year.
The surviving Boston bomb suspect is identified as Dzhokhar A Tsarnaev, 19, of Cambridge, Massachusetts.
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