London bomber spotted by police thanks to his distinctive hat

The teenage refugee had been fostered by a couple in Surrey

A small blast occurred on a busy London tube last week. (AP Photo/Kirsty Wigglesworth)</p>

One of the prime suspects held by British police over the Parsons Green blast was identified by his hat.

The 18-year-old man, thought to be a refugee from the ISIL war zone, was arrested in the coastal town of Dover on Saturday after police officers spotted him wearing a distinctive red cap that matched the one seen in images from the crime scene, The Times reported.

Dover is a gateway to the rest of Europe, and it is believed the suspect was attempting to buy a ferry ticket to the French port of Calais.

CCTV pictures acquired by ITV News show a man wearing a red cap and carrying a Lidl shopping bag in the suburb of Sunbury-on-Thames, Surrey, at 6:50am on Friday morning, just 90 minutes before the attack at the south west London underground station Parsons Green that injured 30 people.

Images posted on social media following the attack appeared to show wires protruding from a flaming bucket contained in a Lidl bag on the floor of the train carriage.

The improvised device, which failed to fully detonate, was packed with fairy lights and is thought to have contained triacetone triperoxide (TATP) explosives, the same powerful explosive used in the 7/7 attacks in London and other deadly bombings across Europe.

The suspect had been fostered by Ronald Jones, 88, and his wife, Penelope, 71, whose house in Sunbury he was seen leaving in the CCTV footage.

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The elderly couple served as foster parents to hundreds of children over several decades, and recently helped refugees from conflict zones including Syria and Iraq. In 2010, they were honoured by Queen Elizabeth II for their work on behalf of children.

According to a neighbour, the couple, who are well respected in the community, had an 18-year-old and a 22-year-old staying with them recently.

Their home was searched by police and remains cordoned off, while nearby houses were evacuated as a precaution on Saturday.

Another man who had previously lived with the couple, Yahyah Farroukh, was also arrested on Saturday in connection with the bombing.

The 21-year-old, believed to be from Syria, was held under the Terrorism Act in Hounslow, west London – a few miles away from Heathrow Airport. His home in Stanwell, Surrey, was later searched.

Both men remain in custody and are being questioned in a south London police station.

Thirty people were taken to hospital after the attack, mostly suffering flash burns, while others were injured in the panicked rush to leave Parsons Green station. It is believed the attack was timed to target a number of young children travelling on the line on their way to school.

Officials say the injuries would have been far worse if the bomb had fully exploded.

On Sunday, Britain’s home secretary Amber Rudd said the recent development in the investigation suggested the attacker was not a “lone wolf”.

Mrs Rudd, who was appearing on the BBC’s Andrew Marr Show, shut down ISIL’s claim that the group was responsible for the attack, saying there was “no evidence” to support this.

Following the arrests, the UK's terror threat level was downgraded from "critical" to "severe," indicating another attack is highly likely but no longer believed to be imminent.

Commuters in London headed back to work on Monday in the first morning rush hour since the attack.

Police have asked the traveling public to be vigilant and to expect to see an increase in armed police.