Attack feels unlike "old school" al-Qaida, but it could be a lone jihadist radicalised online - or something else entirely
• Investigators comb scene in quest for answers
Detectives often talk about the golden hour after an incident – the period in which they can gather the best forensic evidence to help them catch a criminal, or identify the leads that will help them to do so.
In Boston, there should be rich seams for them to mine, even if it takes some time. There will be CCTV footage to go through, debris from the explosives to analyse and witness statements to test for clues.
At the same time, they will be reviewing the intelligence they had prior to the attack across the domestic and international agencies to see if any "flags" were missed; any informers inside groups they are monitoring will doubtless be nudged. If there are suspects, or networks in mind, then phones will be tapped, social media sites monitored.
But beyond the evidence-gathering, investigators will look at what has happened, and ask whether the modus operandi fits any particular group, or suggests one theory as more likely than another. In an information vacuum, such speculation might seem a mug's game for the media, but this is exactly the process the FBI will be going through as it seeks to narrow the boundaries of the inquiry.
"Everything is on the table, until there is good reason to take something off," is the mantra security officials often use.
In the immediate aftermath, there has been speculation that this is a jihadist attack. It's an obvious assumption, though possibly a dangerous one.
But two crude bombs, with no suicide component, and no claim of responsibility, does not look like "old school" al-Qaida – that is, an event set up and co-ordinated by an al-Qaida core outside the US.
Security officials note that if it was an authorised attack from abroad, then this shows how degraded al-Qaida has become over recent years. But at this stage, with the information that has been made public, it does not have this feel.
It is possible that a US citizen travelled abroad and became radicalised. But the nature of the jihadist threat has changed during the years of the wars in Iraq and Afghanistan, and both the US and the UK have become much more prone to self-starter attacks from extremists within. The online magazine Inspire, the work of al-Qaida in the Arabian peninsular (AQAP), has been a particular cause for concern since it was first published in 2010; it has offered advice and encouragement for would-be jihadists to get on with it, and not to wait for training in al-Qaida camps, or orders from above.
The latest edition has just been published and is said to include an "Open Source Jihad (OSJ) section … for aspiring jihadists seeking to assassinate US and European leaders". Though some academics have doubted whether Inspire is such a powerful tool, there is evidence that people inclined to commit violence have been tipped into action by such entreaties.
The Labour MP Stephen Timms was stabbed in his constituency surgery by a British student, Roshonara Choudhry. She had been radicalised listening to and reading the outpourings of Anwar al-Awlaki, then the leader of AQAP.
In Boston, a "drop and run" bombing using relatively unsophisticated – if deadly – explosive devices, without any klaxons sounding from within the intelligence communities, suggests a lone wolf operation, dreamed up and executed by someone not on the police, FBI or CIA radar.
If it is a lone wolf, it doesn't have to have be jihadist inspired. The US government and its military have many enemies, within and without. Oklahoma and Waco have been mentioned as domestic precedents.
A single unhinged person, even one unsure of their motives, can do a lot of damage.
But as one UK official said: "The person behind this could have been home-grown, but whether it is jihadist inspired, or inspired by some other kind of madness, is all just speculation at this stage. You cannot rule anything out."