We do not, as I write, know what lies behind the shooting of a Democratic congresswoman and others in Arizona at a political meeting.

If the YouTube video said to be from the man in custody for the shooting of Gabby Giffords - named by local media as Jared Loughner - really is by him, he seems very disturbed.

Over soft lounge music, with the heading "My final thoughts", text appears mimicking in style formal logic, but in fact making incoherent rambling statements about sleepwalking, numerology and grammar. He writes about the US Constitution, about a currency not backed by gold, and that he can't trust the government and its treasonous laws. But the impression is of unbalanced incoherence rather than political grievance.

While we don't know if the motive behind the shooting was political, it is very clear that it was immediately politicised, at least on the internet. Twitter was immediately full of accusations that the right had created a climate of hatred where this could happen. The right responded in fury. Some pointed out this wasn't based on evidence. One claimed an illegal immigrant was responsible, another that it was done by the left to harm the right.

Much of the criticism was aimed at Sarah Palin.

Congresswoman Gabrielle Giffords had been one of 20 names on a "hit list" issued by Palin, complete with graphics of a rifle-like telescopic sight.

Ms Palin meant, of course, that Ms Giffords was to be targeted by voters in the mid-term elections. The congresswoman herself had warned of the danger of the site, after her office was vandalised in March.

She said: "We're on Sarah Palin's 'targeted' list, but the thing is that the way she has it depicted, we're in the crosshairs of a gun sight over our district. When people do that, they've got to realise that there are consequences to that action."

Ms Palin hasn't directly commented on whether she thinks there were consequences to her actions but she did issue a statement saying: "My sincere condolences are offered to the family of Rep Gabrielle Giffords and the other victims of today's tragic shooting in Arizona.

"On behalf of Todd and my family, we all pray for the victims and their families, and for peace and justice."

After the Oklahoma massacre, Bill Clinton spoke out, linking the attack by extremists to the anti-government rhetoric in Washington. I am uncertain whether it changed the nature of the debate but it was certainly judged something of a turning point for the president.

It is just too soon to say if this shooting will have any lasting impact, although there will no doubt be new calls for a more moderate, less emotional discourse.

But there is an irony.

The rhetoric and debate that instantly sprang up around this crime show the volatile, febrile nature of American politics and those passions are unlikely to disappear overnight.