We must now work on the basis that all is not lost. Many have said that we're back to square one, but that is patently not the case. First of all, Northern Ireland has enjoyed 18 months of peace. That has not yet been breached. Second, all parties have been talking - although not all to each other. You have only to see the difference in the attitudes of Adams and McGuinness to know that much has been achieved. Their cocksure posturing at the start of the ceasefire has been replaced by more thoughtful pronouncements, particularly Adams's statements over the weekend. They now realise that the political process demands more than the megaphone and populist braggadocio.
Their dilemma now is a cruel one. Do they speak for the IRA? If so, and if anyone is to talk further with them, they must condemn the bomb and pledge themselves to an exclusively peaceful way forward. That means condemning those on whose behalf they are the "political" voice; and the IRA would have none of that.
If they say they knew nothing and do not speak for the killers and the bombers, then who needs to talk to them anyway? They claim that they have a mandate and represent 10 per cent of the population who voted for them at the last local elections in Ulster. But that was during the terrorist campaign when the voters knew precisely where they stood: four-square behind the bombers.
Some say that John Major's proposals for elections to a body whose members would then have a mandate to negotiate was the last straw. But how can anyone setting off down the democratic road jib at the prospect of popular confirmation?
There is only one valid reason for some of the objections: that such a body would mark a return to the old, bad days of Unionist-dominated shouting matches at Stormont. I do not believe that is the intention behind the Government's proposals, and anyway they would not go ahead without widespread consent.
The Government must now spell out in more detail what it has in mind and how it hopes to put such fears at rest. This is still the best way forward, though it may now be doomed.
Our American friends may still have an important part to play. For a number of reasons they have more leverage on all shades of nationalism, from the Irish government to the IRA, and might use it to help find a way through the maze.
One thing is sure: handwringing and buck-passing will get us nowhere. We are all responsible and must all put this ghastly weekend behind us and work to ensure it does not recur. Otherwise terrorism really will have won.
The writer is Conservative MP for East Hampshire and former Northern Ireland Minister of State responsible for security.