The man responsible for the Westminster terror attack was previously investigated by MI5. Andrew Douglas assesses the difficulties confronting security services

THE security services will face inevitable questions after it emerged that Westminster attacker Khalid Masood was previously investigated by M15.

Scotland Yard said the Kent-born 52-year-old was not the subject of any current investigations and there was “no prior intelligence about his intent to mount a terrorist attack”.

However, Masood - who had a number of aliases - was known to police and had various previous convictions including GBH, public order offences and possession of offensive weapons, the last being in December 2003.

Whilst he had no conviction for terrorism offences, Prime Minister Theresa May confirmed he was once investigated by MI5 over concerns of “violent extremism” but that he was a “peripheral figure”.

Early indications suggest he acted alone, inspired by “international terrorism” with IS’s Aamaq news agency claiming him a “soldier of the Islamic State”.

The attack bears the hallmarks of a “lone wolf” strike that has long concerned security services.

Since the July 7 bombings in 2005, counter-terror agencies have improved their capacity to uncover major plots - but there now seems a change in tactics.

Cells made up of several people are more vulnerable to communications surveillance and any length of time spent planning a large-scale attack increases the risk of appearing on the authorities’ radar.

But the danger posed by lone individuals adopting low-tech methods such as using knives or vehicles to strike suddenly is much less predictable.

The prospect of single assailants, and the possibility of them being radicalised over the internet, has been repeatedly highlighted by senior security figures.

In December a Europol report said Britain is among the countries “high on the target list” for IS aggression with the scale and impact of “lone actor” attacks increasing.

It added: “The majority of attacks claimed by IS appear to be masterminded and perpetrated by individuals inspired by IS, rather than those who work with the organisation directly.”

Nonetheless, MI5 will face questions over what they knew about Masood and decisions taken when he first appeared on their radar.

It follows a pattern seen in other European countries hit by terrorism in recent years.

Security services in France, Belgium and Germany have all come under the spotlight in the wake of deadly atrocities in Paris, Brussels and Berlin.

However, it is unrealistic to maintain around-the-clock surveillance on all terrorist suspects.

Intelligence agencies estimate there are around 3,000 potentially violent Islamist extremists in the UK, most of them British.

Counter-terrorism units are carrying out more than 500 investigations at any one time and say they have foiled 13 potential attacks in less than four years.

Experts estimate that undertaking permanent surveillance 24 hours a day, seven days a week, on all suspects would require well in excess of 50,000 intelligence officers, dwarfing the number MI5 actually has at its disposal.

Julian King, the European commissioner for security, said “subjects of interest” are assessed by the authorities and often move down extremism watch-lists if their risk level declines.

It makes it almost impossible to eliminate the risk of those people becoming radicalised again and then committing an attack without “plugging in” to a wider terror network where they might be detected.

The UK’s most senior EU official concludes it is “extraordinarily difficult” to prevent attacks like Wednesday’s, even if the terrorist is an individual who at some point was known by police and security services.

“Not everybody who is ever a subject of interest can continuously be a subject of interest.

“The methodology of the London attacker fits into a pattern of behaviour which we’ve seen before, which we witnessed in Nice last year and again in Berlin shortly before Christmas, with the transformation of a vehicle, an everyday object, into a weapon against the softest of targets.

“When we talk about the measures that we are taking together to try and increase our security, I think it’s absolutely crucial that we avoid giving the wrong impression that somehow we can eliminate all the risk, because that’s not right.”