Image copyright AP Image caption Republicans such as Congressman Darrell Issa have held repeated hearings on the Benghazi attacks

There's new evidence, obtained by ABC, that the Obama administration did deliberately purge references to "terrorism" from accounts of the attack on the Benghazi diplomatic mission, which killed four people including the US ambassador to Libya.

Conservatives have long maintained that the administration deliberately suppressed the truth about the attacks.

This is the first hard evidence that the state department did ask for changes to the CIA's original assessment.

Specifically, they wanted references to previous warnings deleted and this sentence removed: "We do know that Islamic extremists with ties to al-Qa'ida participated in the attack."

There's little doubt in my mind that this will haunt Hillary Clinton if she decides to run for president, unless she executes some pretty fancy footwork.

State department spokesperson Victoria Nuland is directly implicated, and the fingerprints of senior White House aides Ben Rhodes and Jay Carney are there as well.

Black and white

Image copyright AP Image caption Republicans are certain to use the Benghazi affair against Clinton should she run in 2016

In the interests of full disclosure I have to say I have not in the past been persuaded that allegations of a cover-up were a big deal. It seemed to me a partisan attack based on very little.

I remember listening to reports from the BBC and others at the time that did suggest the attack in Benghazi was a spontaneous reaction to a rather puerile anti-Islamic video.

I understand President Barack Obama's careful use of the word "terrorism" when it actually means something, rather than as a knee-jerk description of any violence by foreigners against Americans, often in order to justify a "war on terror".

But the evidence is there in black and white, unless we doubt the documents obtained by ABC, which I don't.

Mr Obama's critics are often not very clear what is behind their allegations. I presume they think that the White House wanted to avoid claims the murders were the result of terrorism because this would undermine his claim that al-Qaeda was seriously "degraded". There's also a vague sense he's "soft on terror".

Butt-guarding

The new documents contain two rationales for the changes in language. The first is that it would prejudice the FBI investigation.

Perhaps, but I am not at all persuaded.

The other reason given, old-fashioned butt-guarding, is more credible.

As Ms Nuland puts it, such a report "could be abused by members [of Congress] to beat up the State Department for not paying attention to warnings, so why would we want to feed that either?"

However you read the motives, the state department and apparently the White House did get the CIA to change its story.

This is now very serious, and I suspect heads will roll. The White House will be on the defensive for a while.