Video report by ITV News Security Editor Rohit Kachroo
A teenager is in custody and the home of a respected foster couple has been raided as part of the investigation into the Parsons Green Tube bombing.
Police are searching a house in Sunbury-on-Thames, Surrey, after the 18-year-old was arrested in Dover on Saturday.
Detectives even forced an evacuation of the road as they carried out their search.
The property is said to be registered to a couple awarded MBEs in 2009 for services to children and families.
Neighbours described the couple, Penelope and Ronald Jones, 71 and 88, as "beautiful people" - explaining that they fostered up to seven young people at a time.
Scotland Yard said it was "keeping an open mind" as to whether more plotters were involved in Friday's attack during the morning rush-hour.
Police established an exclusion zone around the Surrey address as neighbours fled their homes.
Dover's ferry terminal was also evacuated when police arrested the teenager.
A tourist described how the suspect was interrogated by unarmed police officers moments before.
Daniel Vaselicu said the suspect appeared calm, "looking normal, not fighting or worried or concerned".
Mojgan Jamali, resident on evacuated Cavendish Road, said she was told by police she had "one minute" to pack her bags, grab her children, and leave her home as police carried out urgent evacuations.
"I was in my house with my children and there was a knock at the door from the police," she said.
"They told me to leave. They said: 'You have one minute to get out of the house and get away.'
"I just got out, I got my three children and we left the house and the street.
The Dover arrest came just over 24 hours after an improvised explosive device injured 30 people on the District Line carriage.
The attack, which was since been claimed by so-called Islamic State, led to the UK terror threat led being raised to critical - the highest level.
Scotland Yard described the arrest as "significant" but warned that the investigation "continues".
"Although we are pleased with the progress made, this investigation continues and the threat level remains at critical," Deputy Assistant Commissioner Neil Basu said.
"The public should remain vigilant as our staff, officers and partners continue to work through this complex investigation.
"We are not, at this time, changing our protective security measures and the steps taken to free up extra armed officers remain in place."
Theresa May announced that the UK's threat level had been raised to critical, meaning an attack was expected imminently.
Armed police and troops have since been deployed across the country as security tightens up.
Highlighting the increased visible police presence, Scotland Yard Commissioner Cressida Dick said: "London has not stopped after other terrible attacks and it will not stop after this one."
Armed police were also seen in many other locations across the UK.
With the threat level raised, Operation Temperer has been enacted.
This allows for the use of the military - up to 5,000 troops - to be deployed in support of the police.
The plan was activated for the first time on May 23 following the Manchester Arena bombing.
Anyone with information should call the Anti-Terrorist Hotline on 0800 789 321