Three people were killed and at least 144 others were wounded when two improvised explosive devices (IEDs) were detonated consecutively at the annual Boston Marathon in the capital of the US state of Massachusetts on 15 April.
The first explosive device detonated among a crowd of spectators near the finish line at 14:50 local time (19:50 GMT). Approximately 12 seconds later, a second device detonated around 100m back along the race course. Media reports, citing an unnamed official, claimed that at least two unexploded devices had been safely disarmed, but this could not be confirmed.
Reuters news agency, citing an unnamed police official, claimed that the IEDs used gunpowder as the explosive and were packed with ball bearings and other shrapnel. While unconfirmed, the claim was consistent with the nature of the wounds reported by hospital authorities.
There was no immediate claim of responsibility, and later on 15 April, President Barack Obama stated: "We still do not know who did this or why," adding: "Any responsible individuals, any responsible groups will feel the full weight of justice."
Nonetheless, at this early stage there are several indicators, albeit circumstantial, which suggest domestic, right-wing extremists were the most likely perpetrators. The attack occurred on Patriots' Day, commemorating the first battles of the American War of Independence, which remains a significant date for right-wing militias, and has been lent added weight by more recent events. Notably, the final assault on the Waco Compound in Texas occurred on Patriots' Day 1993, and the 1995 Oklahoma City bombing - carried out in revenge for Waco - was two days before that year's Patriots' Day.
Other possible factors include the fact that 15 April was 'Tax Day', the deadline for tax returns to be submitted to the federal government, and, as such, a potential motive for anti-government militias.