It is little wonder conspiracy theories about the 1988 Lockerbie airliner bombing continue to flourish. For this was a singularly strange act of terrorism, and not only in its material aspects: the killing of 270 people as the mid-air explosion of Pan Am flight 103 rained debris on a quiet Scottish town.

Even accepting the prosecution case that led to the conviction of Libyan official Abdelbaset Ali Mohmed Al Megrahi - who, apparently a dying man, still protests his innocence - there remains a mismatch of method and motive.

Very well, the forensic trail led back to Libya, via fragments of Maltese clothing and a Swiss bomb timer. Yet the logic of the crime seemed odd.

The proximate reason for the bombing is widely assumed to have been revenge for a 1986 US air raid on Tripoli launched from British soil, which killed an adopted daughter of Libyan leader Colonel Gaddafi; the larger logic being part of Libya’s undeclared war with the United States.

But it was a peculiarly silent kind of revenge: an act of terrorism for which the perpetrator took 14 years to admit any responsibility, and then only obliquely and in return for the lifting of trade sanctions.

This is a type of terrorism quite alien both to the calculating political killers of, say, the 1970s-1990s Provisional Irish Republic Army or the boastful Al Qaeda and Jemaah Islamiyah jihadists to whom glory can be measured in the number of infidel casualties. So much for the notion of terrorism as the propaganda of the deed.

The Conspiracy Files: Lockerbie is a welcome if sometimes frustratingly even-handed effort to sift - perhaps too briskly - through the morass of detail and theories from the long investigation of the Lockerbie bombing.

The explosion brought a welter of speculation about perpetrators and motives, from Iranian revenge for the (however accidental) shooting down of one of its airliners, to claims of a CIA operation gone horribly wrong. Not to mention a false trail via Helsinki and the unsubstantiated theory of a South Africa connection.

And the doubting did not cease with Al Megrahi's 2001 murder conviction. Moreover, his release this year on reportedly compassionate grounds has brought its own fresh round of suspicion, about plots within plots and the sacrifice of justice for international diplomacy now that Libya is in from the cold.

Lockerbie remains ideal fodder for the conspiracy view of history: the idea that events with large impact are typically the consequence of deliberate plans by powerful forces.

It is entirely understandable that many people, especially those who have lost or suffered, are determined to find meaning in disruptive and traumatic episodes of the human story.

But the tragic reality is that there can be a numbing opacity, almost a meaninglessness, even to the most awful human actions and their repercussions.

Before leaping to conclusions about conspiracies, it pays to remember the enormous capacity of individuals and organisations to make stupid errors and misjudgments, to mess up the script of history, to fail to plan or act in ways that are manifestly in their own interests - in short, to surprise us and themselves.

When it comes to investigating or seeking to thwart an act of terrorism, there is so much that simply cannot be assumed: a rational link between intent and outcome; the ability of the group involved to coordinate or even communicate internally; a unity of purpose among its members; or their understanding of the wider effects of the deed.

Yet in this darkness, the counter-terrorist has no choice but to look for clues and patterns.

In the end, the message of Lockerbie, and of this documentary, may be that when it comes to understanding and dealing with terrorism we simply have to settle for partial answers, to take reasonable material precautions, hope for the best and know that sometimes we will fail. Which is cold comfort.