London terror attack: MI5 and police investigating whether further attack is imminent

Detectives piecing together backgrounds of attackers

As they mount the third major investigation in a matter of weeks, the key priority for Britain's counter-terrorism agencies will be to establish whether any further attacks are imminent.

Police believe three terrorists carried out the atrocity and all have been "neutralised" after they were shot dead by armed officers.

Now detectives will be urgently piecing together the backgrounds of the attackers as they examine whether they were part of a wider cell.

In the wake of both the recent Westminster and Manchester atrocities, police made a number of arrests and searched several properties, and it is possible the same pattern will emerge in the coming days.

Investigators will also be scouring CCTV to build as comprehensive a picture as possible of the perpetrators' movements in the lead up to the outrage at London Bridge.

Their electronic communications will come under scrutiny and assessing whether there is any connection to international groups will also form part of the probe.

Picture: (Carl Court / Getty Images)

Counter-terror police and MI5 will be working at pace to identify whether the three assailants plotted the attack alone - with their early theories forming a key plank of any decision regarding the official terror threat level.

It was raised to the most serious rating of critical in the days after the Manchester bombing, before returning to the second highest level of severe last weekend.

The assessment currently remains at severe - meaning an attack is "highly likely".

While assessing whether there is any further threat linked to the men behind the attack will be an immediate priority, security services will also be on high alert for any copycat incidents.

This danger was brought into sharp focus by Prime Minister Theresa May on Sunday.

Theresa May makes her statement following a COBRA meeting in response to last night's London terror attack (Getty Images) (Getty)

She said the three recent attacks are not connected in terms of their planning and execution but warned there is a "new trend" in the threat Britain faces.

"As terrorism breeds terrorism and perpetrators are inspired to attack, not only on the basis of carefully constructed plots after years of planning and training, and not even as lone attackers radicalised online, but by copying one another and often using the crudest of means of attack," she said.

It is also likely that, as soon as they are known, authorities will be running the identities of the attackers through their databases to see whether they were known to them in any way.

The terror threat facing the UK is seen as unprecedented, with 18 plots foiled since 2013 - including five since the Westminster attack in March.

It emerged following the Manchester bombing that as many as 23,000 people have appeared on the radar of counter-terror agencies.

MI5 is managing around 500 active investigations involving 3,000 individuals at any one time, while a further 20,000 have been considered "subjects of interest" in the past.

Press Association