Two cases this week have highlighted one of the most difficult tasks that anti-terrorist agencies face — catching the lone bomber. Andrew Ibrahim, jailed yesterday, and Neil Lewington, convicted on Wednesday, were radically different from one another and yet strikingly similar at the same time.

Ibrahim adopted the ideology of al-Qaeda and wanted to attack the Broadmead shopping centre in Bristol as a symbol of Western decadence. Lewington was a white racist who wanted to bomb Asian and “non-British” targets. Both were loners who proved themselves capable of making deadly explosive devices in their bedrooms.

Ibrahim was caught because his demeanour alarmed the Muslim community. Lewington was thwarted by chance when he was arrested for causing a disturbance on a train.

The history of terrorist investigations shows that the lone operative is often the most difficult to track. Nicky Reilly, another Islamic convert, only failed to commit murder in an Exeter restaurant last year because his bombmaking skills were inadequate.

David Copeland, the London nail bomber, killed and maimed before he was caught, and Theodore Kaczynski, the Unabomber, conducted a 17-year spree in which three died. One anti-terrorist detective said yesterday: “When someone is acting alone, there’s no network to penetrate, no disaffected member of the group who might talk. The more people in the know, the more chance there is of information leaking out.”

Another disturbing feature of Ibrahim’s case was that he was self-radicalised. He discovered Abu Hamza online years after the former imam of Finsbury Park mosque had been jailed. He was to track down Abu Hamza’s sermons and was heard repeating the hook-handed cleric’s description of England as a “dirty toilet”.

The web also provided a bottomless well of al-Qaeda propaganda, including the videoed will of Mohammed Siddique Khan, leader of the July 7 bombers. There were instructions too for making the high explosive HMTD.

“The police spent thousands of hours painstakingly reconstructing Isa Ibrahim’s computer and internet activity in the months leading up to his arrest,” said Moira Macmillan, a CPS lawyer on the case. “The role that his repeated viewing of extremist media played in his radicalisation was central. He spent hours watching and re-watching material on the internet that included videos of suicide bombers and instructions on how to make explosive devices.”

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