The cruelty of Monday night’s attack in Manchester, targeting an audience made up mostly of young teenage girls, is shocking even by the standards of contemporary terrorism.
But it is the fact that it was a bombing rather than a low-tech attack which will cause most alarm among Britain’s counter-terrorism officials.
Apart from the bombings in Paris in 2015 andlast year, most recent terror attacks in have required little planning or co-ordination.
Some, as in Westminster this year and in Nice andlast year, have involved the use of a vehicle to mow down pedestrians.
Others used guns or knives, or as in the murder of Fusilier Lee Rigby four years to the day before the Manchester Arena bombing, a meat cleaver.
The attack in Manchester does not appear to have been as sophisticated as those in Brussels and Paris but even the most rudimentary bombing requires preparation.
The attacker must acquire or manufacture explosives and a detonator and have enough expertise to detonate the device.
Britain’s counter-terrorist intelligence is better than most of its European counterparts but it is impossible to keep all known radicals under constant surveillance.
If the suicide bomber who caused the Manchester attack was already known to the security agencies, they will be alarmed that he was able to assemble a bomb and successfully detonate it without attracting attention.
They must establish if he is part of a broader network and, more urgently, if further, coordinated attacks are planned or likely.
This is not only the deadliest terrorist attack in Britain since the London bombings in July 2005 but the first major incident to take place during a general election campaign.
It came at a crucial moment in the campaign, withunder pressure after a major policy u-turn, while Labour’s poll numbers are rising.
All campaigning has been suspended following the bombing, but the incident is likely to have an impact on both the tone and the content of the campaign.
Moments such as this play to May’s strength as a former home secretary and could present a challenge foras the campaign enters its final fortnight.