At least three dead, many injured in two finish-line blastsRunners were still finishing the race when blasts hitHotline for families of victims: 617-635-4500For witnesses who may have information: 800-494 8477

Summary

This live blog has closed – the following is a summary of events in the immediate aftermath of the Boston Marathon explosions. Further down, the story can be read in chronological order as it unfolded, or you can read a full news story here. Our news coverage of this event continues on the Guardian website.

• At least three people were killed and more than 100 were injured when two explosions struck near the finish line of the Boston Marathon course at about 2.50pm ET. An 8-year-old child was reported to be among the dead. The leading runners had completed the race hours earlier but many runners were still on the course, which remained thronged with spectators.

• Video of the explosions and the immediate aftermath is here. The explosions led to scenes of chaos in downtown Boston, which had been filled with athletes, spectators and merrymakers celebrating Patriots Day, a state holiday.

• "We still do not know who did this or why," President Obama said at a White House briefing three hours after the attack. "And people shouldn’t jump to conclusions before we have all the facts." He referred to "any responsible individuals, any responsible groups."

• Police commissioner Ed Davis denied reports that a suspect was under guard in hospital. But he did say there were a "number of people we are talking to". The FBI has taken the lead on the investigation.

• White House officials called the blasts an "act of terror," although President Barack Obama did not use the word at a briefing. Both Vice President Joe Biden and Boston Marathon officials referred to "bombing" or "bombs." Boston police walked back an initial assertion that a fire at Boston's JFK library was related to the marathon explosions.

• Authorities had fanned out to hospitals to talk with potential witnesses, while a 15-block area around the marathon finish line was sealed off. Hotline numbers were set up for crime tips and for the family members of victims seeking information. The FBI, Department of Homeland Security, national guard and state police joined the Boston police and fire departments and other agencies in responding to the incident and conducting initial investigations.

• Boston residents reached out to athletes stranded in the incident, some of whom were unable to return to their hotels. A database was set up to connect runners in need of food or shelter with local providers. The Red Cross reported ample blood donations.