Here is a timeline of events surrounding the Boston Marathon bombings:
April 15
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.
April 16
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.
April 17
There are reports that a suspect has been taken into custody in connection but this is later denied by the authorities.
April 18
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.
April 19
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. The other is named as his brother, Tamerlan Tsarnaev.
AP