The Islamist behind the Westminster terror outrage was investigated by MI5 as part of a plot to blow up an Army base using a remote-controlled car, The Telegraph can reveal.
Adrian Ajao, 52, who killed four people, including a police officer in last Wednesday's attack, is understood to have been probed six years ago over alleged connections to four al-Qaeda-inspired terrorists.
Zahid Iqbal, Mohammed Sharfaraz Ahmed, Syed Hussain and Umar Arshad, were jailed for a total of 44 years in 2013 after admitting plotting to launch an audacious bomb attack on a Territorial Army base in their hometown of Luton.
Ajao had moved to the town in 2009 following two stints in Saudi Arabia, and lived just a few hundreds yards from one of the ringleaders.
It is thought the fitness fanatic and body builder may have also come into contact with members of the gang when they started preparing for jihad by attending a local gym.
Following last week's attack, Theresa May, the Prime Minister, confirmed that Ajao - who changed his name to Khalid Masood after converting to Islam - had been investigated by the security services for links to "violent extremism".
However after carrying out a risk assessment and looking into his background, it was decided he did not pose a terror threat.
Over the weekend more information began to emerge about Ajao’s recent movements.
As well as having two daughters from an earlier relationship with businesswoman Jane Harvey, with whom he lived in the village of Northiam near Rye in East Sussex, Ajao also has a son and a daughter with his most recent partner, Rohey Hydara.
It is understood they had been living together in Birmingham up until Christmas when Ms Hydara moved back to east London in order to look after her disabled mother.
During his time in Birmingham, the former English tutor had been receiving benefits and neighbours said he did not work.
Last night police confirmed they had made a further arrest in connection with the Westminster attack after raiding a property close to Ajao's Birmingham home.
A 30-year-old man was being held on suspicion of preparation for terrorist acts.
A 58-year-old man who was arrested in Birmingham on Thursday remained in custody, while a 32-year-old woman has been bailed.
Nine other people who had been detained as part of the investigation have now been released with no further action.
Detectives have been working around the clock to try to piece together Ajao's complex background, which saw him transform from a bright, intelligent middle-class schoolboy into a bloodthirsty Islamic terrorist.
While the main focus of the investigation has been to establish if he had any accomplices, police have also been looking into any links with extremists in the past.
It has now emerged that during his time living in Luton, Ajao was a close neighbour of Taimour Abdulwahab, the Swedish student who blew himself up in Stockholm after becoming radicalised when he attended university in the Bedfordshire town.
He also lived just yards from Abu Rahin Aziz, the jihadi who was killed in a drone strike in the Isil stronghold of Raqqa in Syria in 2015.
But it is thought it was his association with the gang behind the remote-controlled car plot that first put him on the radar of the security services.
In 2010 MI5 began monitoring four Luton-based extremists after learning that they might be preparing for some sort of attack.
As part of a sophisticated surveillance operation, agents bugged the men's vehicles and eventually overheard Iqbal, a married father of two, discussing driving a toy car carrying explosives under the gates of the town's Territorial Army base.
In one conversation he was heard saying to his accomplice, Ahmed: "At the bottom of the gate, there’s quite a big gap. If you had a little toy car, it drives underneath one of their vehicles or something.”
The group were also overheard discussing plans to attack MI5 headquarters, a US Air Force base, an English Defence League gathering and a local shopping centre.
As part of their preparations Ahmed, who lived less than a mile from Ajao, began attending a local gym.
He later led a series of military-style training trips to Snowdonia and the Brecon Beacons where they were monitored as they jogged in formation and used logs as mock weapons.
After months of surveillance, the men's homes were raided and they were all arrested in September 2011, just days before the tenth anniversary of the 9/11 attacks.
They were jailed in April 2013 after admitting having been inspired by al-Qaeda.
Mr Justice Wilkie QC said the men posed a significant risk to the public and jailed the ringleaders, Iqbal and Ahmed, for 16 years and three months each.
Arshad received six years and nine months and Hussain was given five years and three months.
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