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Moments after the horror attack the clandestine world of counter-terrorism was hurled into a high-tempo behind-the-scenes hunt for the killer’s identity and connections.

Every agency from British special forces, MI6, our domestic intelligence spooks at MI5 and all of the spy agencies in America and across Europe are involved in the hunt.

A British Special Forces team attached to the Police Counter Terrorist Command spearheaded the search of the House of Commons for any other killers and oversaw the Prime Minister’s evacuation.

The SAS team, based at New Scotland, bundled Theresa May to safety, along with her usual security detail.

Minutes after the attack a fast-travelling RAF Chinook helicopter crew at RAF Odiham in Hampshire was alerted ready to fly in and evacuate the PM out of the capital in case of a second attack.

For live updates on the suspected terror attack, click here

(Image: Reuters)

The SAS has more than 20 specialists attached to Counter Terror Command with an additional immediate reaction force permanently on call in and around London to respond.

Other Army officers, headed by a Major attached to MI5’s Joint Terrorist Analyst Cell (JTAC) in central London were on standby to send more troops via the Director of Special Forces.

A special codeword was sent from the JTAC to the office of the Director of Special Forces, where the operations officer put on “high alert” an SAS “immediate reaction" team.

(Image: Reuters)

Inside Downing street the PM’s senior military liaison officer is a serving SAS man in touch with all of the above agencies.

Over the past two years the Police and Special Forces immediate response force has mounted numerous exercises across the UK to test their plans in case of a major incident.

Meanwhile within minutes of the attacks MI5’s London HQ a mile away was in total lockdown - with nobody going in to the intelligence agency’s Thameside building except for intelligence officers.

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An emergency police hotline to MI5’s Thames House building informing senior officers an attack was underway triggered a well-rehearsed and sadly, often launched emergency plan.

A liason team of MI5 specialists was dispatched to police Counter Terror Command to serve up a live feed on the identity of the attacker and his history.

An operations room was set up with intelligence officers monitoring terrorist targets’ communications, their mobile phones, e-mail systems and internet apps to hoover up “chatter.”

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Terror attacks always spark chatter between jihadists officers and the aftermath is often the best time to plug into their shadowy world to listen to what they are saying.

A former MI5 officer told the Daily Mirror: “Every government building, police HQ and security building like MI5 will be in lockdown for some time and they will be busy.

“Early impressions are that this could be exactly the kind of attack we all feared - one by an apparent loner who may have attracted little or no interest from the counter-terrorism community.

(Image: Getty Images Europe)

“Of course it’s early to make an assessment but if it was a lone wolf attack, they are the worst as they are far less likely to trigger to alarm within the local community or in phone calls to friends.

“All of the usual mechanisms that would set of alarm bells with my former colleagues are not triggered if someone is not talking about their intention to be involved in an attack.

“But other reactions are also very telling. There may an interception of a telephone call in which targets who are under technical surveillance are inspired by the news of a new attack.

(Image: Getty Images Europe)

“It may mean there is a heightened risk of copycat attacks elsehwhere in the United Kingdom or against British targets abroad.”

Most importantly MI5 officers were working throughout the night following the attack, deploying surveillance teams and reassessing targets for surveillance in a race to shut down future attacks.

Surevillance teams and intelligence officers throughout the UK are re-assessing operations and making sure they had not overlooked the killer’s links to other people.

(Image: PA)

His family and known associates were also under heavy scrutiny and police were tracking them down, possibly trying for arrest warrants if there is the slightest suggestion they were involved.

At Britain’s huge listening station Government Communications HQ - or GCHQ - spooks were on high alert for intercepted communcations by jihadists globally to see if that raises fresh intelligence.

And liason officers at both GCHQ and MI5 were contacting opposite numbers within the US CIA, National Security Agency and other western intellignece agencies to swap information.

(Image: EPA)

Our former MI5 officer source told the Daily Mirror: “It is of course a terrible tragedy that this has happened but it could help unearth a trove of intelligence about plots against Britain.

“The death toll could have been so much higher but practically the jihadists had a result today - there are horrific images being beamed throughout the world, showing that terror has reached Britain once again.”