A 21-year-old, identified by his employers as Yahyah Farroukh, was arrested after he finished his shift at a fried chicken shop in Hounslow,on Saturday (September 16) night as part of the probe into Friday's attack.
According to his Facebook profile, Farroukh is originally from Damascus and is living in London, having studied English at West Thames College.
His profile says he works as a nightclub promoter.
Both Farroukh and an 18-year-old man, understood to be the suspected bomber, are believed to have spent time in foster care with Penelope and Ronald Jones, aged 71 and 88 respectively, who previously received MBEs for services to children and families.
Suleman Sarwar, 43, who owns Aladdin's Fried Chicken in Kingsley Road, Hounslow, with his brothers, said Farroukh worked at the takeaway.
Mr Sarwar said: "Yes he is familiar. I recognise him as a member of staff.
"He was very normal. I don't know how long he worked here.
"It was surprising seeing him on the news."
Mr Sarwar said Farroukh had been working on Saturday and was arrested outside the shop after his shift at around 11.30pm.
He said: "It was at that point that the police all grabbed him."
Asked if police had been staking out the shop, Mr Sarwar said "I suspect so" but said he was not aware of any operations.
The takeaway owner added he was sure Farroukh was Syrian because of his distinctive dialect.
Mr Sarwar said police had taken CCTV recordings from the shop on Sunday (September 17) and carried out a search of the property.
A spokesman for West Thames College, in Isleworth, said: "West Thames College confirms that Yahyah Farroukh, who, according to media reports is the second suspect detained in connection with the Parsons Green incident, was a former student at the college from December 2013 to June 2015.
"Yahyah Farroukh joined the college aged 17 to learn English and completed ESOL Threshold courses.
"The college has robust, well-established Prevent procedures and will co-operate fully with requests from the police."
The younger suspect was detained on Saturday morning in the departure area of Dover ferry port, which is the busiest ferry hub in Europe and a gateway to the French coast.
Both men remain in custody for questioning over the attack, which injured 30 people.
Farroukh's home in Stanwell, Surrey, was searched by police on Sunday, after armed officers raided the Jones' home in Sunbury-on-Thames on Saturday.
A local politician said he understood an 18-year-old Iraqi orphan was living with the couple, having moved to Britain aged 15 after his parents died.
Leader of Spelthorne Borough Council, Ian Harvey, whose ward is Sunbury East, said he learnt about the boy's background from neighbours of Mr and Mrs Jones and information available publicly.
He told the Press Association: "One thing I understand is that he was an Iraqi refugee who came here aged 15 - his parents died in Iraq."
Of the other suspect, he added: "I think it is widely known that this person who lives at (the Stanwell) property was a former foster child at the property which was raided yesterday."
Home Secretary Amber Rudd on Sunday said the UK’s threat level has been lowered from critical to severe. She added: “It still means an attack is highly likely, so I would urge everybody to continue to be vigilant but not alarmed.”
Assistant Commissioner Mark Rowley, the country’s top counter-terrorism officer, said police were gaining a “greater understanding of the preparation of the device”.
Security services are trying to find how the Iraqi teen entered the country and who he had associated with.
Just one person was last night still being treated in hospital following the bomb attempt – for serious wounds to their lower body.
The Metropolitan Police said a 21-year-old man was arrested in Hounslow at around 11.50pm on Saturday (September 16).
He was arrested under section 41 of the Terrorism Act and taken to a south London police station where he remains in custody.
The 18-year-old man was arrested by Kent Police in the port area of Dover on Saturday under section 41 of the Terrorism Act.
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