Within minutes of the explosions in Boston, the procedures required for a successful investigation will have been put in place.
The first responders treated the injured and bystanders pulled away wreckage from those trapped.
But following in quickly behind them will have been the men and women who were already thinking ahead.
The scene was quickly sealed off. Every piece of debris will need to be collected and analysed.
Somewhere among the thousands of shards of metal and glass there will be answers to the first questions: What sort of bombs were they? Did they have timers? Were they detonated by remote control?
Any remnants of the bombs will be examined in detail and all footage of the event - professional and amateur video as well as CCTV - will be scrutinised.
The latter will be watched to see if anyone can be seen acting suspiciously at any point - in the hours, minutes and seconds leading up to the explosions.
Experts will also be painstakingly searching through social media to see if there is anything that their trained eye can spot. That is a very labour intensive procedure.
The FBI will also look at the records of mobile phone companies to isolate all the calls made in the area.
They will want to see if the same phone made two calls in quick succession in the few seconds before the two explosions.
They will also cross refer the phone records with any calls made by anyone they bring in for questioning.
This is all basic modern intelligence work, but most of it will not answer another important question - what was the motive?
Because America has suffered so many terrorist attacks of a different nature, the investigators will have to consider several possibilities.
The US has experienced attacks inspired by foreign Islamist jihadist philosophy, but it also has a long history of home-grown terrorism.
In recent decades it has seen the sporadic attacks of the Weathermen in the 1970s, bombings of abortion clinics beginning in the 1980s and the Oklahoma bombing in the 1990s which killed 168 people.
Behind the Oklahoma attack was the idea that "Big Government" had to be taken on by violent means.
The idea of "The State" is deeply unpopular in parts of the US especially among the survivalist militias.
Timothy McVeigh, who was convicted for Oklahoma , came from that mindset and regarded himself as a true patriot.
It may be a coincidence that the Boston bombing came on Patriots Day, the annual event commemorating the first major battle of the Revolutionary War.
But the date will have been noted by the authorities as one potential avenue to explore.
Until things become clearer there are many such potentials, and although the men and women in the FBI will have their gut feelings about motive, they will have to keep an open mind.