Last July the UK's top policeman, Sir Bernard Hogan-Howe, literally knocked on wood when asked the likelihood of another terror attack in the capital.

At a briefing for journalists including Fairfax, he admitted "we are worried there may be attacks that get through".

"It could happen to us tomorrow," he said.

In the end, it appears to have happened eight months later - just after his retirement.

There had been no successful terror attack in London since the May 2013 murder of soldier Lee Rigby in South London.

In the same period, countries across Europe have been rocked by multiple, deadly attacks.

But the terror threat level in the UK has been at severe - meaning an attack is 'highly likely' - since 2014, when it was raised in response to the rise of Islamic State in Iraq and Syria.

There is only one higher level, 'critical', which means an attack is imminent.

Last year the Daily Mail reported there were 2000 terror suspects in the UK.

So if, as police suspect, this was a terror attack, the questions to ask today are threefold.

Of course, we must know why this happened, who is behind it. Police are working on it.

But there is a third, more heartening question: how was something like this prevented for so long?

Hogan-Howe put it down to the UK having the best intelligence services in the business.

"We tend to arrest people before the attack and on the whole at the moment they have not got through - but only because of good intelligence," Sir Bernard said.

Just about every week, there is another report of an arrest and a charge on terror offences. It sometimes doesn't even make the news. Here is a sample from this month.

Just two days ago in Cardiff a 34-year-old pleaded guilty to five terrorism charges.

He had been an active member of IS, sitting in his bedroom developing a website that taught other IS members and supporters how to hack into computers for criminal purposes.

He even had a USB stick disguised as a cufflink which he used to hide his activities.

Two weeks ago a 22-year-old from Acton in west London pleaded guilty to 'preparing terrorist acts'. He had made a concerted effort to get to Syria to join IS.

On March 7, a 30-year-old man from south London was arrested on terror charges.

On March 2, a 45-year-old from north-west London was charged with three counts of disseminating terrorist material.

On March 1, the Met's counter-terror command arrested a 35-year-old man on "suspicion of inviting support" for IS.

Since 2013 there has been a consensus in the halls of power that one day this intelligence net would fail.

With so much relentless pressure, one day, someone would get through.

Today, it appears, they did.

Tonight at least three families will mourn their dead.

There will be hard questions to ask, about why the UK's police and intelligence services didn't spot this one, or at least prevent it.

All this said - a big caveat is needed.

At the time of writing this is not a confirmed terror attack. The motives, state of mind, background and mental health of the attacker have not been publicly reported.

Police say they "remain open-minded to the motive" - but a full counter-terrorism investigation is already underway, led by the Met's Counter Terrorism Command.

Police are reviewing their priorities and resources across the capital, as of this minute.

Already eyes are looking, not at this threat, but forward to the next.

And the next.

And the next.