There is a witch-hunt vibe in Labour on how and who approved Mandelson's appointment

The question being asked everywhere today is "how did it happen"? Because the vibe out of Downing Street this morning seems to be that nobody anywhere did anything wrong, processes were followed, and everything went by the book.

But can they really, honestly, believe that?

To recap, the reason that everyone is asking is to try and discern whether the failings are a consequence of a fundamental, unfixable flaw at the heart of Keir Starmer's operation.

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Yesterday, we told you that the security services had raised red flags about the appointment of Peter Mandelson, yet Number 10 went ahead.

The story was nuanced. We did not say that Peter Mandelson had failed a deep vetting, just that concerns were relayed and the appointment went ahead.

We put the story to Downing Street, and - being candid - I did not understand what their official response meant, beyond it quite obviously not being a denial.

As a response, Number 10 said to us that the security vetting process is all done at a department level - with no Number 10 involvement.

To a wider group of political journalists, an hour and a half after we aired the story, Number 10 said they were "not involved in the security vetting process. This is managed at the departmental level".

Today, the line from Downing Street seems to be that there was no official level block on the appointment, so it went ahead.

Although The Times has reports from allies of Lord Mandelson claiming he disclosed everything, the exact chain of events remains opaque.

But for those who want to understand the inner workings of government, here is more detail about the two types of check that would have gone on, and what this tells us.

Firstly, by the security services.

The Cabinet Office led both on vetting and separately on propriety and ethics (a form of government HR) but in effect, it's multi-agency and multi-department.

In this instance, potentially multiple agencies would likely feed into the Foreign Office, or FCDO.

FCDO then act as a liaison for vetting - what I'm told is known as a "front face" - and an FCDO official takes a note to tie everything together.

We are being told that this amounts to a binary decision.

So, potentially, an FCDO official ties up the findings from both agencies and departments in one place and that's given to the Permanent Under Secretary at the department (Philip Barton, later Olly Robbins) and Number 10.

So the recommendations can both be "by a Foreign Office official" and from security services at the same time. That potentially explains some reporting this morning.

I believe, ultimately, I was told about the security service red flags because they do not want to share the blame for a catastrophic intelligence miss that has harmed this government severely.

And is a situation like this ever binary? If there are matters of judgement for the PM to weigh up, are we honestly saying they are kept from him?

Sources tell me there are always conversations around the side of these processes: it would be recklessly incurious of Number 10 if this had not been the case for someone who already resigned twice and whose association with Jeffrey Epstein was in the public domain.

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But then there is a second, Cabinet Office-led process which is arguably more important.

There will have been checks on Lord Mandelson by examining what's in the public domain.

It is, quite simply as one person said to me, a "Google check".

This, too, must have flagged stories about Lord Mandelson's relationship with Epstein post-conviction, and gone to Number 10.

At this point, the question is why No 10 did not see the sheer enormity of the risk this posed and pressed ahead anyway.

Who thought this was okay, and why?

There is a witch-hunt vibe to the Parliamentary Labour Party right now.

Now - and forever - there will be footage of Sir Keir Starmer in the Commons chamber defending keeping an ally in place who admitted a close relationship with a known paedophile after conviction and a jail sentence, before sacking him the next day.

The previous week, he was defending another ally who had avoided tax, before sacking her two days later.

The damage is likely to be immense.