A teenager suspected of planting the Parsons Green bomb was arrested in the departure area at the port of Dover, Scotland Yard said.
The 18-year-old was detained on suspicion of preparing a terrorist act, sparking an evacuation of the ferry terminal, Deputy Assistant Commissioner Neil Basu said.
Detectives investigating the bombing were said to be “keeping an open mind” about whether more plotters were involved and Mr Basu said the terror threat would remain at its highest level, indicating another attack was feared.
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He added: “Officers from Kent Police had to partially evacuate the port of Dover at around 11.40am this morning and this was to enable them to search the premises as a precautionary measure.
“That work is now complete and they have recovered a number of items during that search.”
A separate search is under way at a house in Sunbury-on-Thames, Surrey – home to foster care couple Penelope and Ronald Jones, who received MBEs for services to children and families. The surrounding houses were evacuated by counter-terror officers.
The suspect is now being held at a London police station after being arrested by Kent Police.
Mr Basu said: “At this stage we are keeping an open mind over whether more than one person is responsible for the attack and we are still pursuing numerous lines of inquiry and at great pace.”
Echoing the earlier words of Home Secretary Amber Rudd, he added: “All I am prepared to say at the moment is it is a very significant arrest, but as I said we are open-minded. If there are other people responsible it’s our job to find them and that is part of the reason that we are remaining at critical threat.”
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The Surrey property is registered to Mr and Mrs Jones, 88 and 71, who were honoured by the Queen in 2009.
Several residents said the couple lived there and Nicola Ryder, who lives opposite, described them as “beautiful people”. She said she knew them just as Penny and Ron, and that they would foster up to seven young people at a time between the ages of 10 and 18.
Police on Cavendish Road, Sunbury-on-Thames (Victoria Jones/PA)
On the street, residents were rushed from their homes by armed police and are now facing the prospect of not returning before Sunday.
Mother-of-three Mojgan Jamali, among those forced to flee, told the Press Association: “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.”
Nearby Staines Rugby Club has been opened as a rest centre for evacuees, with an estimated 150 to 200 people already said to have been welcomed through its doors.
Thirty people were injured when the improvised device partially exploded on a District Line train at Parsons Green station during the Friday morning rush hour. Three victims are still being treated at Chelsea and Westminster hospital, NHS England said.
Earlier the Home Secretary said it was “much too early to say” whether the bomber was part of the current security picture, following comments from US President Donald Trump that the culprit was known to Scotland Yard.
A key strand of the investigation has focused on CCTV as officers comb through footage to establish who planted the device, and when and where it was placed on the train.
The suspect was arrested at around 7.50am, in the port which is the busiest ferry hub in Europe and serves as a commercial gateway to the French coast, including Calais and Dunkirk.
Police have since identified 121 witnesses, of whom 100 have been spoken to, Mr Basu said.